Stepped light guide illumination systems

ABSTRACT

A stepped light guide illumination system employing a planar sheet of an optically transmissive material having a stepped light guiding structure and one or more parallel arrays of channels. Light is input into the stepped light guiding structure using a number of light sources, such as LEDs, located within the channels and coupled to the stepped light guiding structure at multiple locations. Light is extracted from the stepped light guiding structure by two-dimensional patterns of light extraction features provided within the parallel channels and in surface areas of the stepped light guiding structure located between the parallel channels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 62/514,946 filed on Jun. 4, 2017, incorporated herein by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to illumination devices employing planarlight guides. More particularly, this invention relates to wide-arealuminaires, lighting panels, illuminated panel signs, front lights,backlights, backlighting units, electronic displays, backlit displayscreens, advertising displays, road signs, decorative broad-area lights,as well as to a method for redistributing light from discrete lightsources in such devices.

2. Description of Background Art

Conventionally, wide-area light emitting devices employ planar lightguides, also commonly referred to as “waveguides”, which are illuminatedfrom one or more edges using Light Emitting Diodes (LEDs) or other typesof compact light sources. The conventional edge-lit illumination systemsmay exhibit certain limitations such as difficulty to efficiently coupleenough light into edges to provide sufficient light output from arelatively large area of the light guide. Additionally, guiding lightlong distances from light input edges is associated with optical lossesin the light guide material which may lead to energy waste andsuboptimal performance of the illumination device.

U.S. Patent Applications Publications No. US-2014-0226361-A1 (the '361Publication) and US-2017-0045666-A1 (the '666 Publication), thedisclosure of which is incorporated herein by reference in its entirety,disclose face-lit waveguide illumination systems formed by a planarwaveguide and optical coupling elements attached to a face of thewaveguide. U.S. Pat. No. 9,097,826 (the '826 patent) the disclosure ofwhich is incorporated herein by reference in its entirety, disclosesillumination systems employing a planar light guide having cylindricallenses formed in a broad-area light output surface of the planar lightguide and extending perpendicular to a light input edge. U.S. Pat. No.9,256,007 (the '007 patent), the disclosure of which is incorporatedherein by reference in its entirety, discloses illumination systemsemploying planar light guides associated with lens arrays, includingarrays of cylindrical lenses and the like.

BRIEF SUMMARY OF THE INVENTION

Certain aspects of embodiments disclosed herein by way of example aresummarized in this Section. These aspects are not intended to limit thescope of any invention disclosed and/or claimed herein in any way andare presented merely to provide the reader with a brief summary ofcertain forms an invention disclosed and/or claimed herein might take.It should be understood that any invention disclosed and/or claimedherein may encompass a variety of aspects that may not be set forthbelow.

According to one embodiment, a stepped light guide illumination systemis exemplified by a first sheet of an optically transmissive material, aplurality of smaller-area second sheets of an optically transmissivematerial attached to a broad-area surface of the first sheet, a firstplurality of light extraction features formed in a surface of the firstsheet, a second plurality of light extraction features formed in asurface of the second sheet, and a plurality of light sources opticallycoupled to a light input edge of at least one of the plurality of thesecond sheets. A broad-area surface of the first sheet may include aplurality of lenses extending perpendicular to the light input edge.

According to one embodiment, a stepped light guide illumination systemis exemplified by a sheet of an optically transmissive material having aplurality of channels formed in a broad-area surface of the sheet, aplurality of light extraction features formed in that or oppositesurface of the sheet and defining edges extending perpendicular to theplane of the sheet, and a plurality of light sources located within oneor more of the plurality of channels and optically coupled to the edges.According to different implementations, the channels may be arrangedinto a parallel array or two or more parallel arrays crossed at an anglewith respect to each other. Bottom portions of the channels may includea plurality of light extraction features. A broad-area surface of thesheet may include a plurality of lenses extending perpendicular orparallel to the channels.

According to different implementations, the light sources may includelight emitting diodes (LEDs), lasers or other types of compact lightsources. The light extraction features may be formed by discretemicrostructures distributed over an area according to a predefinedtwo-dimensional pattern and configured for extracting light from thestepped light guide illumination system. A sheet of reflective materialmay be positioned on one side of the sheet of an optically transmissivematerial.

According to one embodiment, a method of making a stepped light guideillumination system consistent with the present invention, includesproviding a large-area sheet of an optically transmissive material,cutting a series of parallel channels in the sheet, forming a pluralityof light extraction features in a surface of the sheet, and positioninga plurality of LEDs within the channels. The method may further includepositioning a reflective sheet on one side of the sheet of an opticallytransmissive material.

According to one embodiment, a method of making a stepped light guideillumination system consistent with the present invention, includesproviding a first sheet of an optically transmissive material, attachinga plurality of smaller-area second sheets of an optically transmissivematerial to a broad-area surface of the first sheet, forming a pluralityof light extraction features in a surface of the first and secondsheets, positioning a plurality of LEDs within spaces between individualsecond sheets the channels, and attaching a reflective sheet to surfacesof the second sheets.

Various implementations and refinements of the features noted above mayexist in relation to various aspects of the present inventionindividually or in any combination. Further features, aspects andelements of the invention will be brought out in the following portionsof the specification, wherein the detailed description is for thepurpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view and raytracing of a stepped lightguide illumination system, showing multiple planar light-guiding sheetsattached to a surface of a larger planar light guiding sheet andmultiple light sources coupled to edges of the sheets, according to atleast one embodiment of the present invention.

FIG. 2 is a schematic perspective view of an alternative configurationof a stepped light guide illumination system, showing multiple rows andcolumns of planar light-guiding sheets, according to at least oneembodiment of the present invention.

FIG. 3 is a schematic bottom view of a stepped light guide illuminationsystem, showing an alternative arrangement of light guiding sheets,according to at least one embodiment of the present invention.

FIG. 4 is a schematic bottom view of a stepped light guide illuminationsystem, showing a two-dimensional array of rectangular light guidingsheets or sections distributed over a base light guiding sheet,according to at least one embodiment of the present invention.

FIG. 5 is a schematic bottom view of portion of a stepped light guideillumination system, showing a two-dimensional array of hexagonal lightguiding sheets or sections distributed over a base light guiding sheet,according to at least one embodiment of the present invention.

FIG. 6 is a schematic bottom view of portion of a stepped light guideillumination system, showing a two-dimensional array of stepped lightguides separated from each other by spacing distances, according to atleast one embodiment of the present invention.

FIG. 7 is a schematic section view and raytracing of portion of astepped light guide illumination system, showing additional opticallayers over a stepped light guide, according to at least one embodimentof the present invention.

FIG. 8 is a schematic section view of portion of a stepped light guideillumination system, showing side-emitting LEDs optically coupled toedge surfaces of optically transmissive sheets and further showing lightextraction features formed on a surface of a base light guiding sheetand on surfaces of light guiding sheets that are attached to the basesheet, according to at least one embodiment of the present invention.

FIG. 9 is a schematic section view and raytracing of portion of astepped light guide illumination system, showing a multi-chip LED andmultiple light coupling layers, according to at least one embodiment ofthe present invention.

FIG. 10 is a schematic bottom view of a portion of a stepped light guideillumination system, illustrating an exemplary configuration of an areaof optical contact between two planar light guiding sheets opticallycoupled to each other, according to at least one embodiment of thepresent invention.

FIG. 11 is a schematic bottom view of a portion of a stepped light guideillumination system, illustrating an alternative exemplary configurationof an area of optical contact between two planar light guiding sheetsoptically coupled to each other, according to at least one embodiment ofthe present invention.

FIG. 12 is a schematic section view and raytracing of portion of astepped light guide illumination system, showing a light input edgesurface inclined an angle with respect to a normal to a prevalent planeof a stepped light guide and further showing a light collimating elementassociated with an LED source, according to at least one embodiment ofthe present invention.

FIG. 13 is a schematic section view and raytracing of a portion of astepped light guide illumination system, showing a tapered light inputedge of a light guiding sheet that has a greater thickness than the restof the light guiding sheet, according to at least one embodiment of thepresent invention.

FIG. 14 is a schematic view showing an exemplary distribution pattern oflight extraction elements for a portion of a stepped light guideillumination system, according to at least one embodiment of the presentinvention.

FIG. 15 is a schematic view showing an exemplary distribution pattern oflight extraction elements for a larger portion of a stepped light guideillumination system including alternating areas having different surfacedensities of the light extraction elements, according to at least oneembodiment of the present invention.

FIG. 16 is a schematic section view of a portion of a stepped lightguide illumination system, showing a reflective light diffusing sheetcoupled to a bottom surface of a top light guiding sheet, according toat least one embodiment of the present invention.

FIG. 17 is a schematic section view of an exemplary configuration of astepped light guide illumination system selected for raytracing,according to at least one embodiment of the present invention.

FIG. 18 is a graph showing exemplary dependencies of a relative surfacearea or surface density of light extraction features from a distancefrom a center of a stepped light guide.

FIG. 19 is a graph showing exemplary calculated dependencies of asurface luminance from a distance from a center of a stepped lightguide.

FIG. 20 is a schematic bottom view and raytracing of a portion of astepped light guide illumination system, showing a single row of LEDsilluminating different light guiding sheets, according to at least oneembodiment of the present invention.

FIG. 21 is a schematic bottom view and raytracing of a portion of astepped light guide illumination system, showing corrugated light inputedges of light guiding sheets, according to at least one embodiment ofthe present invention.

FIG. 22 is a schematic section view of a stepped light guideillumination system, showing a stepped light guide having a monolithic,single-piece body, according to at least one embodiment of the presentinvention.

FIG. 23 is a schematic section view of a stepped light guideillumination system, showing an array of rounded ridges or lenses formedin a light emitting surface of a stepped light guide, according to atleast one embodiment of the present invention.

FIG. 24 is a schematic perspective view and raytracing of a steppedlight guide illumination system, showing a parallel array or roundedridges or lenses formed in a light output surface of a planar lightguide, according to at least one embodiment of the present invention.

FIG. 25 is a schematic perspective view and raytracing of a steppedlight guide illumination system, showing an array of highly elongatedoptical couplers formed in a surface of a planar light guide and anarray or light sources disposed in registration with and coupled toterminal ends of the optical couplers, according to at least oneembodiment of the present invention.

FIG. 26 is a schematic perspective view and raytracing of a steppedlight guide illumination system, showing wedge-shaped, highly elongatedoptical couplers, according to at least one embodiment of the presentinvention.

FIG. 27 is a schematic section view of a stepped light guideillumination system portion, showing dimensions of a planar lightguiding sheet, an optical coupler, an LED source, a cylindrical lens anda series of light extraction features, according to at least oneembodiment of the present invention.

FIG. 28 is a schematic section view of an alternative configuration of astepped light guide illumination system portion, showing a tapered lightinput end of a light guiding sheet, according to at least one embodimentof the present invention.

FIG. 29 is a schematic perspective view of a stepped light guideillumination system portion, showing an optical coupler formed in twoopposing broad-area surfaces of a light guiding sheet, according to atleast one embodiment of the present invention.

FIG. 30 is a schematic section view of a stepped light guideillumination system portion, showing an optical coupler formed by asymmetrical tapered end of a light guiding sheet, according to at leastone embodiment of the present invention.

FIG. 31 is a schematic perspective view of a light input edge portion ofa light guide, showing an array of first surface corrugations formed ina light input edge surface of the light guide and two arrays of secondsurface corrugations formed in light input faces of optical couplers,according to at least one embodiment of the present invention.

FIG. 32 is a schematic section view of a stepped light guideillumination system portion, showing an array of light extractionfeatures formed in a surface of a first sheet of an opticallytransmissive material in an area where a second sheet of an opticallytransmissive material is attached to the first sheet, according to atleast one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the system generally shown in thepreceding figures. It will be appreciated that the system may vary as toconfiguration and as to details of the parts without departing from thebasic concepts as disclosed herein. Furthermore, elements represented inone embodiment as taught herein are applicable without limitation toother embodiments taught herein, and in combination with thoseembodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection and distribution of electromagnetic radiant energy,such as light, in a broad spectrum or any suitable spectral bands ordomains. Therefore, for the sake of simplicity of expression, withoutlimiting generality of this invention, the term “light” will be usedherein although the general terms “electromagnetic energy”,“electromagnetic radiation”, “radiant energy” or exemplary terms like“visible light”, “infrared light”, or “ultraviolet light” would also beappropriate.

Furthermore, many applications exist for the present invention inrelation to distributing light by means of a planar optical light guidewhich hereinafter may also be referenced to as a waveguide. The planaroptical light guide (waveguide) refers to a broad class of objectsemploying an optically transmissive material confined between twoopposing broad-area surfaces that are substantially parallel to eachother. The term “substantially parallel” generally includes cases whenthe opposing surfaces are parallel within a reasonable accuracy. It alsoincludes cases when the body of the material defined by the broad-areasurfaces has a slightly tapered shape (the taper not exceeding 1 angulardegree) or has a slightly varying thickness across the surface. It yetfurther includes cases when a generally planar body of the light guideincludes limited areas where its thickness is different compared to therest of the light guide.

It is also noted that terms such as “top”, “bottom”, “side”, “front” and“back” and similar directional terms are used herein with reference tothe orientation of the Figures being described and should not beregarded as limiting this invention in any way. It should be understoodthat different elements of embodiments of the present invention can bepositioned in a number of different orientations without departing fromthe scope of the present invention. In the context of the description ofa planar light guide and its elements, the term “top” is being generallyused to refer to a primary light emitting side of the light guide andthe term “bottom” is being generally used to refer to the opposite side(which may or may not be non-emitting) for the sake of convenience ofdescription and not in a limiting sense.

According to the present invention, the planar light guide may beexemplified by a transparent plate, sheet, slab, panel, pane,light-transmitting substrate or any suitable sheet-form of an opticallytransmissive material, including film thicknesses and rigid and flexiblesheet forms. This invention is also applicable to any two-dimensionalshape variations of the sheet forms, including but not limited to asquare, rectangle, a polygon, a circle, a strip, a freeform, or anycombination therein. This invention is further applicable to anythree-dimensional shapes that can be obtained by bending the sheet formsaccordingly, including but not limited to cylindrical orsemi-cylindrical shapes, conical shapes, corrugated shapes, and thelike.

According to the present invention, there is provided an illuminationsystem employing an optical light guide exemplified by an opticallytransmissive, broad-area panel, which may also hereinafter be referredto as a “light guiding panel” or “LGP”. The LGP is made from a materialwhich has a refractive index greater than that of the outside medium andis capable of guiding light within the panel by means of a TotalInternal Reflection (TIR) from its opposing broad-area surfaces,provided that the internal incidence angles onto either of the surfacesare greater than a critical angle of TIR characterizing the broad-areasurfaces.

For the purpose of this discussion, the term “incidence angle” of alight ray in relation to a surface generally refers to an angle thatsuch ray makes with respect to a normal to the surface. It will beappreciated by those skilled in the art of optics that, when referringto light or other forms of electromagnetic waves passing through aboundary formed between two different refractive media, such as air andglass, for example, the ratio of the sines of the angles of incidenceand of refraction is a constant that depends on the ratio of refractiveindices of the media (the Snell's law of refraction). The followingrelationship can describe a light bending property of an interfacebetween two refractive media: n₁ sin ϕ_(I)=n_(R) sin ϕ_(R), where n_(I)and n_(R) are the respective refractive indices of the materials formingthe optical interface and ϕ_(I) and ϕ_(R) are the angle of incidence andthe angle of refraction, respectively. It will be further appreciatedthat such optical interface can also be characterized by a critical TIRangle which is the value of ϕ_(I) for which ϕ_(R) equals 90°.Accordingly, for a surface characterized by a stepped drop in refractiveindex along the propagation path of a ray, the incidence angle may beless than, equal to, or greater than the TIR angle at the given surface.

A TIR angle ϕ_(TIR) can be found from the following expression:ϕ_(TIR)=arcsin(n _(R) /n _(I)·sin 90°)=arcsin(n _(R) /n _(I))  (Equation1)

In an exemplary case of the interface between glass with the reflectiveindex n_(I) of about 1.51 and air with n_(R) of about 1, ϕ_(TIR) isapproximately equal to 41.5°.

It will be appreciated that, once light is input into the LGP and itspropagation angles permit for TIR to occur at LGP's longitudinal walls,the light becomes trapped in LGP and can propagate considerabledistances until it is extracted, absorbed or reaches an edge of thepanel, for example.

The present invention will now be described by way of example withreference to the accompanying drawings.

FIG. 1 depicts a stepped light guide illumination system 900, accordingto an exemplary embodiment of the invention. Stepped light guideillumination system 900 includes a generally planar light guide 800 thatis formed by a first substantially planar base sheet 10 of an opticallytransmissive material and a plurality of second sheets 20 of anoptically transmissive material attached to the base sheet 10. Theorientation of planar light guide 800 and its components athree-dimensional space may be conveniently described using orthogonalreference axes X, Y, and Z (see FIG. 1) which also define orthogonalreference planes XY, XZ, and YZ.

Sheet 10 has a rectangular configuration and is defined by opposingbroad-area surfaces 11 and 12 and four edge surfaces 13, 14, 15 and 16.Surfaces 11 and 12 extend parallel to each other and extend broadly bothlongitudinally and laterally along the X and Y axes so as to form aplanar sheet form that is parallel to the XY plane. Opposing edgesurfaces 13, 14 are parallel to each other, extending parallel to the XZplane, and opposing edge surfaces 15, 16 are likewise parallel to eachother, extending parallel to the YZ plane.

Each sheet 20 is defined by opposing broad-area surfaces 21 and 22 thatare parallel to the plane XY and has for edge surfaces, including edgesurfaces 23 and 24, which are parallel to the plane XZ, and edgesurfaces 25 and 26, which are parallel to the plane YZ. Each sheet 20has a highly elongated, rectangular configuration with a longitudinalaxis extending along the X direction.

Each of sheets 10 and 20 has a non-zero thickness so that light guide800 has a total thickness defined by a combined thickness of sheets 10and 20. Referring to FIG. 1, the thicknesses of light guide 800 andsheets 10 and 20 may be conventionally measured along the Z axis (or Zcoordinate). According to one embodiment, the thickness of each ofsheets 10 and 20 is sufficiently low such that the respective sheets arehighly flexible. The term “flexible”, as used herein with respect tooptically transmissive sheets, is directed to mean that the respectivesheet(s) can be flexed with a relative ease without breaking and couldnormally be returned to the original, e.g., planar, shape or state aftersuch flexing. The term “highly flexible” is directed to mean that thesheet(s) can be easily flexed to a curved shape such that a prevalentradius of curvature of the sheet can be comparable to or less than adimension of the sheet along which such flexing occurs. For example, asheet can be considered highly flexible when it can be easily bent suchthat its opposing ends are parallel to each other (corresponding to a180-degree bend) or when a ratio between a length and a curvature radiusis of the order of 2, 3 or more.

Sheet 10 extends continuously along the entire width of light guide 800and illumination system 900 (as measured along the X axis). Sheet 10 ispreferably formed from a highly transmissive, solid dielectric materialand is configured to guide light both longitudinally and laterally inresponse to optical transmission and TIR from opposing surfaces 11 and12. Surfaces 11 and 12 are preferably optically smooth and polished to ahigh gloss. Edges surfaces 13, 14, 15 and 16 may also be polished andconfigured for reflecting light by means of TIR with high efficiency.One or more edge surfaces 13, 14, 15 and 16 may also be covered with aspecularly reflective mirror or a diffuse reflector. For example, suchedge surfaces may be coated with a metallic layer (e.g., aluminized orsilvered). In another example, strips of a highly reflective material,such as a metallized film or foil may be applied to any of the edgesurfaces. The film or foil may alternatively have a matte,light-diffusing surface.

Similarly, each sheet 20 is made from a highly transmissive material andconfigured to guide light longitudinally and laterally in response tooptical transmission and TIR from opposing surfaces 21 and 22. Its edgesurfaces 23, 24, 25 and 26 may be polished for promoting TIR. One ormore of the edge surfaces may be at least partially coated or covered bya reflector. The reflector may be of a specular type or a diffuse type.

Each sheet 20 is attached to broad-area surface 11 of sheet 10 so thatsurfaces 11 and 22 form a good physical and optical contact. It may bepreferred that surfaces 11 and 22 are mated or bonded together using anoptical adhesive or encapsulant to form a single, monolithic structurewith good optical coupling and preferably with refractive indexmatching. Each sheet 20 may be optically coupled to sheet 10 withrefractive index matching along the entire extent of surface 22.Alternatively, sheet 20 may be optically coupled to sheet 10 along oneor more portions of surface 22. For example, only a central area ofsurface 22 may be optically coupled with index matching and peripheralareas of surface 22 may be separated from surface 11 by a thin layer ofair of a layer of optically absorptive or reflective material.

The term “optically coupled” is directed to mean any relationshipbetween two optical components which enables light to pass from oneoptical component to the other without being rejected, reflected back orabsorbed. The term “refractive index matching”, in relationship to anoptical coupling, is directed to mean such optical coupling between tworefractive optical components where the refractive components are matedtogether with a tight optical contact, directly or through one or moreintermediate optical layers, and without an air gap.

In the context of the present invention, index matching may serve acongruence of goals. One particular goal of the index matched opticalcoupling is reducing or eliminating the Fresnel reflections at therespective optical interface in a broad range of incidence angles.Another goal may be eliminating or suppressing TIR in an exemplary casewhere the refractive index of a first optical element is less than thatof the second optical element and where the light is expected topropagate from the first element to the second element. In this case,the proper index matching should ensure that no TIR will occur at therespective optical interface within the designed range of incidenceangles and that essentially all of the light (e.g., >95%) will betransmitted to the second optical element. It should be understood thatindex matching generally precludes forming an air gap between the tworefractive optical elements which could impede the light passage.

For the purpose of this invention and from the practical standpoint, twooptical elements may also be considered optically coupled with indexmatching when they are disposed in a tight optical contact, directly orthrough one or more intermediate optical layers, and when thelight-receiving optical element has a greater refractive index than thelight-emitting component or, if lower, when the differences between therefractive indices are less than a predefined, sufficiently smallamount.

According to one embodiment, each sheet 20 may be glued to surface 11 ofsheet 10 using an optically clear adhesive which refractive index isapproximately equal to or slightly greater than the refractive index ofthe material of sheet 20. The adhesive should preferably have arelatively high tack and be capable of forming a permanent bond. It isalso preferred that the adhesive layer is sufficiently thin so as not tosignificantly add to the thickness of the resulting structure. Accordingto one embodiment, sheets 20 may be coupled to sheet 10 using one ormore intermediate optically clear solid substrates or film-thicknesssheets having refractive indices similar to those of sheets 10 and 20.Layers of an optically clear adhesive may be used to bond each of therespective substrates or layers together.

According to one embodiment, the adhesive layer between sheets 20 and 10may be formed by a two-sided optical adhesive tape or sheet. In oneembodiment, it may be formed by a layer of curable optical materialapplied in a liquid form with the subsequent curing. Suitablenon-limiting examples of such adhesives include UV-curable,moisture-curable, or two-component silicones or acrylic-base UV-curableresins conventionally used for mating optical sheets or layers in LCDdisplays. It may be preferred that the refractive index of the adhesivelayer is very closely matched to the materials of sheets 10 and 20,e.g., within 2-3%.

Suitable materials for making sheets 10 and 20 include but are notlimited to water-clear (low-iron) glass, Poly(methyl methacrylate) (PMMAor acrylic), polycarbonate, styrene, cured urethane, polyester,silicone, and the like. According to one embodiment, sheets 10 and 20are made from the same or very similar material (e.g, the same-grade ordifferent grades of acrylic). According to one embodiment, sheets 10 and20 are made from different materials which nevertheless have similarrefractive indices n, such as, for example, acrylic (n≈1.49) and certaingrades of glass having the refractive index of around 1.5. The adhesivewhich is used to bond such sheets may have a refractive index between1.47 and 1.53.

Each sheet 20 has a length L₂₀ and a width W₂₀ that is considerably lessthan length L₂₀. According to different embodiments, width W₂₀ is lessthan length L₂₀ by at least 1.5 times, at least 2 times, at least 3times, at least 5 times, and at least 10 times. Sheets 20 aredistributed over surface 11 with a spacing distance S₂₀ between thenearest edges of adjacent sheets 20. Such spacing distance S₂₀ ispreferably much less than W₂₀. According to different embodiments,spacing S₂₀ is less than width W₂₀ by at least 2.5 times, at least 3times, at least 5 times, and at least 10 times.

It is noted that the illustrated position and orientation of sheets 20with respect to the edges of sheet 10 should not be constructed aslimiting this invention in any way. Sheets 20 may also be positioned atany other location of surface 11 and may have identical or differentspacing from each other and/or from edges of sheet 10. One or moresheets 20 may be located at and aligned flush with any edge of sheet 10.According to the embodiment illustrated in FIG. 1, sheets 20 arearranges into a linear array with the outermost sheets 20 in the arraylocated at a distance from the nearest edge. According to oneembodiment, the outermost sheets 20 may be disposed flush with thenearest edge. Sheets 20 may also be disposed in any other suitableorientation with respect to sheet 10, e.g., diagonally.

According to an aspect of the embodiment depicted in FIG. 1, steppedlight guide 800 includes a plurality of sections 54 that are formed bysheets 20 and respective sections of sheet 10 disposed below sheets 20(or above sheets 20, depending on the orientation). Such stepped lightguide 800 further includes a plurality of sections 55 of sheet 10 thatare free from sheets 20. Accordingly, each section 54 of stepped lightguide 800 has a greater total thickness than each section 55 since suchtotal thickness is defined by a sum of thickness of sheets 10 and 20.The transitions from sections 54 to sections 55 are characterized by astepped drop in the thickness of stepped light guide 800, thus forming“steps” in the respective broad-area surface of the light guide. Sincesections 54 and 55 are alternating with each other in a repeatingpattern, stepped drops in a thickness of stepped light guide 800 arealternating with stepped increases in the light guide thickness suchthat stepped light guide 800 still retains a generally planarconfiguration.

According to an aspect of the present invention, stepped light guide 800is formed by base sheet 10 and a plurality of light coupling elementsexemplified by sheets 20 attached to a broad-area face (surface 11) ofthe base sheet. In operation, light that is input into sheets 20 is thencoupled into base sheet 10 through its broad-area face (surface 11)rather than through one or more of its edges. Accordingly, certainembodiments of stepped light guide 800 represent a novel variation of aface-lit waveguide illumination system. Further explanation of theprinciples of light input through a face of a light guide (waveguide) aswell as several functional examples of face-lit waveguide illuminationsystems can be found in the '361 and '666 Publications the disclosure ofwhich is incorporated by reference herein in its entirety.

Stepped light guide 800 may have a fairly large number of sheets 20attached to a single, continuous base sheet 10 and may thus have a largenumber of alternating sections 54 and 55. According to differentembodiments, stepped light guide may extend up to several meters bothlongitudinally and laterally, with its size being limited mostly by amaximum practical size of continuous sheet 10. According to differentembodiments, continuous stepped light guide 800 may include 2 sheets 20,3 sheets 20, 4 sheets 20, 5 to 10 sheets 20, and more than 10 sheets 20.

According to one embodiment, multiple stepped light guides 800 may bepositioned adjacent to each other in the XY plane to form even larger,sectioned illumination panels. Such multiples light guides 800 may beseparated by small air gaps or joined together at their respective edgesurfaces (such any one, two, three or all of edge surfaces 13, 14, 15and 16).

According to an aspect, the plurality of light guiding sheets 20distributed over surface 11 forms a plurality of parallel channels 80each having a rectangular cross-section and extending from edge surface15 to opposing edge surface 16 of sheet 10. A width of such channels 80is defined by and is thus equal to spacing S₂₀. Parallel channels 80 mayform a regular array distributed over the entire broad area of the bodyof light guide 800.

Referring yet further to FIG. 1, stepped light guide illumination system900 further includes a plurality of light sources exemplified by LEDs 2.LEDs 2 are distributed over surface 11 in a two-dimensional array andarranged in rows and columns. Each row in the array includes a series ofLEDs 2 arranged into an individual LED strip and optically coupled toone or more edges of sheet 20. More specifically, each LED 2 ispositioned in a close proximity to respective edge surface 23 or 24 ofindividual sheet 20. Each LED 2 is also oriented with its light emittingaperture towards the respective light input surface so thatsubstantially all of the light emitted by the LED is coupled to therespective sheet 20. It is generally preferred that at least 80% oflight emitted by LEDs 2 is coupled to sheets 20 through the respectiveedge surfaces 23 and/or 24.

Each of the edge surfaces 23 and 24 is associated with a strip of LEDs 2such that each sheet 20 has at least two strips of LEDs illuminating thesheet from opposing edges. If any of edge surfaces 23 and 24 is providedwith a mirrored surface or is covered with a strip of reflectivematerial, openings or window areas for LEDs 2 should be provided withinsuch mirrored/reflective surfaces for an unimpeded light passage fromLEDs 2 to sheet 20.

LEDs 2 of at least their light emitting apertures are positioned withinchannels 80. Accordingly, a width of each channel 80 (same as spacingS₂₀) should be sufficient to accommodate at least one or two rows ofLEDs 2. Accordingly, spacing S₂₀ should be greater than the respectivedimension(s) of LEDs 2 (as measured along the Y axis).

Surface 11 of sheet 10 further includes a plurality of light extractionfeatures 8. Light extraction features 8 are formed in the areas that arefree from sheets 20 (including sections 55 and portions of surface 11that are located within channels 80). The areas of the physical contactof sheets 20 with surface 11 (sections 54) are preferably free fromlight extraction features 8. In other words, light extraction features 8are preferably formed only in the spaces between sheets 20 or areassurrounding sheets 20.

Light extraction features 8 may occupy substantially the entire exposedarea of surface 11 and may also extend all the way to opposing edgesurfaces 13 and 14. According to an aspect of the embodiment of FIG. 1,surface 11 has light extracting areas (patterned with light extractionfeatures 8) that are alternating with spacing areas (which are generallyfree from light extraction features 8) in a repeating pattern. Thespacing areas are represented by wide strips (extending along the Xaxis) which are generally free from light extraction features 8 andwhich have width W₂₀. The light extraction areas are represented bynarrower strips (extending along the same X axis) that include lightextraction features 8 and that have width S₂₀.

Light extracting features 8 may include any suitable optical elements orsurface features configured for intercepting and extracting light fromsheet 10. Light extracting features 8 may be configured to extract lightby means of scattering, reflection, refraction, deflection, and/ordiffraction. Light extracting features 8 may be further configured toextract light while changing one or more properties of light. Exemplaryproperties of light that may be changed by light extracting features 8include but are not limited to a wavelength, polarization, spectraldistribution, angular and/or spatial distribution, and dispersion.

According to one embodiment, light extracting features 8 includerelatively small dots of a highly reflective, light scattering materialdeposited to surface 11 according to an ordered or randomtwo-dimensional pattern. Suitable materials for such light extractingfeatures 8 may include white inks or paints having a reflectance of atleast 80% in the visible spectrum, preferably having at least 85%reflectance, even more preferably at least 90% reflectance, and stilleven more preferably at least 95% reflectance. The light scattering dotsmay be formed by white inks that are UV-curable, aqueous (water-based)or solvent-based. In a non-limiting example, the light scattering dotscan be printed on surface 11 using a flatbed material depositionprinter, an ink-jet printer or a screen printer. The white ink mayinclude nanoparticles of titanium dioxide or other type of white,high-reflectance powder suspended in a liquid resin or suspension whichviscosity is suitable for the selected type of surface depositiontechnique.

According to one embodiment, the dots of light scattering material maybe microdots that are invisible or barely visible by a naked eye from anormal viewing distance of 40-60 centimeters. By way of example, themicrodots may have sizes on a scale of several micrometers to around 100micrometers. According to one embodiment, the microdots may have sizesfrom 50 micrometers to 150 micrometers. The microdots may also haveirregular shapes or a combination of regular (e.g., round or oval) andirregular shapes.

The thickness of microdots may vary across the surface of the respectivesheet. According to one embodiment, the thickness of the material insuch microdots may increase with the distance from a light input edge(e.g. edge surface 13). According to an alternative embodiment, suchthickness may decrease with the distance from the light input edge. Thevariability of the thickness of the light scattering material from onemicrodot to another may be combined with the increase or decrease of thedensity of microdots across surface 11, for example, to obtain a desiredlight emission pattern or to homogenize the luminance of surface 11and/or surface 12 across the entire light-emitting area of stepped lightguide 800.

According to one embodiment, surface-deposited light extraction features8 may include materials that have specific color-filtering or colorconverting properties and can change the color of light upon lightstriking such light extraction features. Light extracting features 8 mayinclude a mix of such color-filtering or color converting materials,each material being different from the other(s), such that the opticalproperties of the emitted light can be altered in more than one way.According to one embodiment, light extracting features 8 may haveluminescent properties. For example, the resin or suspension used toprint light extraction features on surface 11 may include a luminescentmaterial or phosphor that converts shorter wavelength of light in the UVor visible spectrum into longer wavelengths in the visible range. Suchmaterial may be configured to absorb light in a first wavelength andre-emit light in a second wavelength which is different than the firstwavelength. According to one embodiment, it is preferred that the secondwavelength is greater than the first wavelength. By way of example, suchmaterial may be configured to absorb at least a portion of blue lightemitted by some types of LEDs and re-emit the energy of such blue lightin the form of perceptibly white light. By way of example and notlimitation, the luminescent material may be configured to convert 350nm-400 nm UV light from a “black light” into visible wavelengths.

Light extracting features 8 may be distributed over the designatedarea(s) of surface 11 according to an ordered or random pattern.According to one embodiment, the ordered pattern may be formed by a twodimensional regular array of rows and columns. In one implementation,every other row or every other column may be shifted relatively to theadjacent rows or columns so as to form a staggered array or rows orcolumns. According to one embodiment, the positions of individual lightextracting features 8 may be randomized within an otherwise orderedpattern. According to one embodiment, light extraction features aredistributed according to a high-density pattern and have a cumulativearea that is greater than 50% of the exposed area of surface 11.According to one embodiment, substantially the entire exposed area ofsurface 11 may be coated by a continuous layer of a light diffusingmaterial, such as white ink or bulk scattering particles suspended in apolymeric material, for example.

According to one embodiment, light extraction features 8 are formed by alight scattering or light diffusing film that is attached to surface 11in the respective areas (e.g., in sections 55 and within channels 80).Such film should preferably have a hemispherical reflectance of at least85%, more preferably at least 90%, and still more preferably at least95%.

LED chips employed in LEDs 2 may be configured to emit a blue light.Light extracting features 8 may be configured to change the lightemission spectrum upon interaction with blue light propagating instepped light guide 800. For example, a YAG phosphor may be employed inlight extracting features 8 to convert such blue light to a white light.The phosphor material may be mixed with silicone or other material.Light extraction features 8 may be deposited directly to surface 11 in aliquid form, for example, by printing, spraying, dispensing, coating orother suitable methods. Two or more different phosphor materials may becombined to independently control the emission spectrum. Theconcentration of each phosphor material may be varied from one lightextraction feature 8 to another, for example, to obtain various colorpatterns or effects for different areas of stepped light guide 800. In anon-limiting example, different areas of stepped illumination system 900may be configured to emit light in different colors in response todifferent concentrations of one or more phosphor materials in thoseareas.

According to one embodiment, light extraction features 8 are formed bylight-deflecting or light-diffusing surface microstructures formed insurface 11. The microstructures may include ordered or random surfacerelief features formed, for example, by means of etching, embossing,laser ablation, sanding, micromachining, micro-replication and any othermethod suitable for producing the desired surface texture or relief.

According to one embodiment, each light extraction feature 8 may beconfigured to emit light according to a symmetric or quasi-symmetricangular pattern with a peak emission towards a normal direction withrespect to surface 11 or towards a direction that deviates from thenormal direction by less than 10 degrees. According to one embodiment,the angular pattern may be highly asymmetric, e.g., exhibiting a peakemission towards a direction that makes an angle with respect to thesurface normal of greater than 20 degrees, greater than 30 degrees,greater than 45 degrees or greater than 60 degrees.

According to some embodiments, light extraction features 8 may be formedby micro-cavities formed in surface 11. Each micro-cavity may have theshape of a lens, a prism, a groove, a blind hole, or can be simply amicroscopic discontinuity in surface 11 allowing some light to escapefrom light guide 800 in the respective location. According to oneembodiment, light extraction features 8 may be formed by highlyelongated grooves. The highly elongated grooves may longitudinallyextend parallel to edge surface 13, perpendicular to edge surface 13 orat an angle with respect to such surface. According to one embodiment,each highly elongated groove may have near vertical walls and a lengththat is at least 2.5 times, at least 3 times, at least 3.5 times, atleast 4 times, and at least 5 times greater than a transversal width ofthe groove (the length and width of the groove being measured in the XYplane).

According to some embodiments, light extraction features 8 may be formedby micro-bumps formed in surface 11. Each micro-bump may have the shapeof a lens, a prism, or an irregular protrusion in the surface, allowingat least some light to escape from light guide 800 in the respectivelocation. According to one embodiment, each light extraction feature 8may include a microscopic recess or cavity in surface 11 and an adjacentmicroscopic protrusion in the surface. For example, individual lightextraction feature 8 may be formed by a round micro-crater in surface 11encircled by a round rim protruding away from surface 11.

The size of individual light extraction features may range fromsubmicron sizes up to several millimeters. According to one embodiment,the size of individual light extraction features 8 is between 1micrometer and 20 micrometers. According to one embodiment, the size ofindividual light extraction features 8 is between 20 micrometers and 100micrometers. According to one embodiment, the size of individual lightextraction features 8 is sufficiently small and the spacing betweenindividual features is sufficiently large so that stepped light guide800 has a substantially transparent appearance at least along a normal(perpendicular) viewing direction.

According to one embodiment, light extraction features 8 may be formedin a separate film or thin-sheet material which is then applied tosurface 11 with a good optical contact and preferably with indexmatching. According to one embodiment, light extraction features 8 maybe formed in surface 12. According to one embodiment, light extractionfeatures 8 may be formed both surfaces 11 and 12, for example, toenhance the light extraction rate.

Referring yet further to FIG. 1, each light guiding sheet 20 issimilarly provided with light extraction features 9 formed in surface21. According to one embodiment, light extraction features 9 may be ofthe same or similar type as light extraction features 8. Any and allfeatures described above in reference to light extraction features 8 maybe applied, without limitations and in any combination, to lightconfiguring extraction features 9. According to one embodiment, theoptical properties of individual light extraction features 8 and/or 9may be varied over the areas of respective sheets 10 and/or 20. Lightextraction features 8 may also be formed in surface 22 of each sheet 20.

According to some embodiments, certain characteristics of lightextraction features 9 or their two-dimensional pattern may be differentfrom those of light extraction features 8, including, for example, arelative area, spatial density, color or spacing between individuallight extraction features. The differences may include optical,geometrical, mechanical, chemical and any other type of properties. Thedifferences in properties may also be on the level of individual lightextraction features 8 and/or 9.

According to different embodiments, light extraction features 9 may becharacterized by different size, thickness, reflectance, absorption,luminescence, fluorescence, light scattering or light deflectionproperties compared to light extraction features 8. If features 8 and 9are formed by surface microstructures, the size, geometricalconfiguration and/or slopes of light deflective facets or sides offeatures 9 may be different from those of features 8. Any of theabove-discussed properties of features 8 and/or 9 may also vary acrosssurfaces 11 and/or 21, in any combination. According to one embodiment,light extraction features 9 may be formed by one type of lightdeflecting elements (e.g., inkjet-printed microdots) while lightextraction features 8 may be formed by a different type of lightdeflecting elements (e.g., surface microstructures formed bymicroimprinting or hot embossing). According to one embodiment, eitherone or both sheets 10 and 20 may include bulk scattering particlesembedded into sheet's material and configured for deflecting lightpropagating in the light guiding mode.

The patterns of light extraction features 8 and 9 are configured toprogressively extract light propagating in the respective sheets oflight guide 800 and result in a substantially uniform light emissionfrom surface 12. In order to achieve this, each of the respectivepatterns of light extraction features 8 and 9 may have a variablespatial density which is generally increasing with the distance fromLEDs 2. Such variable spatial density can be a function of the size andthickness of the respective sheets 10 and 20 and can be determined fromoptical raytracing experiments. Sections 54 and 55 of stepped lightguide 800 may have different densities of light extraction features 9and 8, respectively. The spatial density, geometrical parameters and/oroptical characteristics of light extraction features may also varyindividually within each of the sections 54 and 55.

A uniformity U of luminance of surface 12 may be defined by thefollowing relationship: U=1−(L_(PEAK)−L_(AVG))/L_(AVG), where L_(PEAK)is a peak luminance and L_(AVG) is an average luminance characterizingthe surface. The peak luminance may be measured using spot measurementsat different locations of surface 12 using a spot luminance meter. Thesampling area for spot measurements may be defined by a circular areacharacterized by a radius that is much smaller than the X and Ydimensions of light guide 800. Depending on the circumstances, apreferred size of the sampling area may also be defined by thecharacteristics of the measurement tool, the overall size of the panelor the intended application (for example, the anticipated viewingdistance). According to one embodiment, the size of the sampling area issmaller than S₂₀ and is preferably much smaller than S₂₀. In otherwords, the spot measurements should preferably have sufficientgranularity to measure surface luminance variations on a scale that isless than the size of a gap between sheets 20.

According to one embodiment, luminance uniformity U of surface 12 is atleast 70%, more preferably at least 80%, even more preferably at least85%, and yet even more preferably at least 90%. According to oneembodiment, a difference between an average luminance of stepped lightguide 800 in sections 54 and 55 is less than 30%, more preferably isless than 25%, even more preferably is less than 20%, even morepreferably is less than 15%, and still even more preferably is less than10%.

According to an aspect of the embodiment depicted in FIG. 1, steppedlight guide 800 is formed by two distinct light guiding layers whichrepresent different pieces of optical sheet form material. One layer isformed by continuous light guiding sheet 10 and the other layer isformed by a series of smaller-size light guiding sheets attached tosheet 10. Accordingly, light guide 800 has a stepped configurationhaving a variable thickness. The longitudinal edges of sheets 20 (edgesurfaces 23 and 24) define transition areas where light guide 800 hasstepped (abrupt) changes in its thickness.

It should be understood that light sources illuminating the light inputedges (e.g., opposing edge surfaces 13 and 14) are not limited to lightemitting diodes (LEDs) and may include any other suitable single ormultiple light sources of any known type, including but not limited to:fluorescent lamps, incandescent lamps, cold-cathode or compactfluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide,electroluminescent lamps or sources, field emission devices, lasers,etc. Each light source may have a linear configuration and may include asingle linear light-emitting element or a relatively small number oflinear light-emitting elements. Each light source may also two or morecompact light emitting elements incorporated into a linear array or anarrow two-dimensional cluster. When the light source includes multiplelight emitting elements, each of the light emitting elements may have acompact shape or an extended two-dimensional or one-dimensional(elongated) shape.

LEDs 2 may be further associated with integrated or external optics suchas collimating or light-redistributing lenses, mirrors, lens arrays,mirror arrays, light diffusers, waveguides, and optical fibers. Whenmultiple light emitting elements are employed, each of the lightemitting elements may be provided with individual optics. Alternatively,a single linear optic may be provided for the entire array to collimatelight or otherwise shape the emitted beam in a plane which isperpendicular to the longitudinal axis of the array.

According to different embodiments, light may be input into steppedlight guide 800 through one, two, three or all four edge surfaces 13,14, 15 and 16, for example, for maximizing the light output from surface12.

Stepped light guide illumination system 900 may include a cover ofhousing configured or encase LEDs 2, channels 80 and optionally encasesheets 20. The housing may also encase or frame the exposed portionsand/or edges of sheet 10. Such housing may have different functionsincluding but not limited to structural, protective (from dust,moisture, elements, impact, etc.) and/or aesthetic.

According to an aspect of the embodiment depicted in FIG. 1, steppedlight guide 800 defines three distinct parallel planes by surfaces 21,11 and 12. Light extraction features 9 and 8 are formed in two of thesedistinct planes (in surfaces 21 and 11, respectively) while light isprimarily output from light guide 800 through the third surface (surface12).

The overall dimensions of stepped light guide illumination system 900,the sizes and shapes of sheets 10 and 20, the types of LEDs 2 as well astheir number, spacing and nominal power may be selected based on thetarget application. According to one embodiment, stepped light guideillumination system 900 may be configured as a flat-panel lightingluminaire. According to another embodiment, it may be configured as abacklight unit of an LCD display. According to a yet another embodiment,it may be configured as a backlight for an illuminated sign, artwork orimage print. According to a yet another embodiment, it may be configuredas a backlight for a photobioreactor. In any of such applications, asuitable housing may be provided (not shown), which may be designed tocover the non-emitting side of stepped light guide 800 and also enclosesheets 20 and channels 80 with LEDs 2.

Stepped light guide illumination system 900 may further optionallyinclude additional light sources optically coupled to edges of lightguiding sheet 10. In FIG. 1, such additional light sources areexemplified by a pair of LEDs 322 coupled to edge surface 13. However,it should be understood that any other number of LEDs 322 may be used.Furthermore, such LEDs 322 may be coupled to two, three, or all fouredges of sheet 10.

FIG. 2 schematically illustrates an embodiment of stepped light guideillumination system 900 in which sheets 20 have different rectangularshapes and arranged according to a different pattern over surface 11.LEDs 2 of FIG. 2 also have a different arrangement compared to FIG. 1.Referring to FIG. 2, stepped light guide 800 includes one sheet 20 thathas the same dimensions as sheets 20 depicted in FIG. 1 and further hasfour other rectangular sheets 20 having different dimensions. The fourother sheets 20 are identical to each other, each having a rectangularconfiguration, and are arranged into a regular two-dimensional arrayhaving two rows and two columns. Each of sheets 20 of FIG. 2 isassociated with four linear arrays or LEDs 2 coupled to exposed edges ofthe sheets. Each of edge surfaces 23, 24, 25 and 26 of each sheet 20 isprovided with an array of LEDs 2.

It is noted that stepped light guide 800 may have any suitable number ofsheets 20 which may have different shapes and may be arranged in anysuitable manner. Similarly to the above-described embodiments of FIG. 1,each sheet 20 of FIG. 2 defines section 54 having a larger thicknesswhile spacing areas between sheets 20 define a plurality of sections 55of a reduced thickness. The two-dimensional array of sheets 20 defines afirst series of parallel channels 80 that are oriented parallel to the Xaxis and further defines a second series of channels 80 that are alignedparallel to the orthogonal Y axis.

Stepped light guide 800 may include any other mix of different shapesand sizes of sheets 20. This is further illustrated in FIG. 3 whichschematically depicts a yet another arrangement and other shapes ofbottom light guiding sheets attached to surface 11 of top base sheet 10.Referring to FIG. 3, a bottom light guiding sheet 20′ has a highlyelongated rectangular shape having a length L₂₀₁ and a width W₂₀₁, whereL₂₀₁ is greater than 2.5 times W₂₀₁. A longitudinal axis of sheet 20′extends parallel to the X axis. A pair of sheets 20″ and 20′″ haveelongated rectangular shapes each having a length L₂₀₂ and a width W₂₀₂.The longitudinal axes of sheets 20″ and 20′″ extend parallel to the Yaxis. W₂₀₂ is greater than W₂₀₁ and L₂₀₂ is significantly less thanL₂₀₁. Rectangular sheets 20″″ and 20′″″ both have an elongated shapewith a length L₂₀₂ and a width W₂₀₃, respectively, and are orientedparallel to the X axis.

FIG. 3 further illustrates different geometrical configurations andpatterns of LEDs 2. Sheet 20′ is completely surrounded by LEDs 2′ whichare distributed around the perimeter of the sheet with approximatelyequal spacing. Sheets 20″ and 20′″ are illuminated by two-sided LEDs 2″″which are configured to emit light towards opposing sides and thusilluminate both sheets simultaneously. Sheet 20′″ is further illuminatedby two highly elongated LEDs 2″ that cover almost the entire area of therespective light input edge. Sheet 20″ is further illuminated by asingle highly elongated LED 2 which length is about equal to L₂₀₂.Sheets 20″″ and 20′″″ are illuminated by LEDs 2′ coupled to select edgesand having different spacing.

According to one embodiment, at least some or all of sheets 20 may havean identical, substantially square shape. FIG. 4 schematically shows atwo-dimensional array of square light guiding sheets 20 distributed oversurface 11 of base sheet 10. Densely populated LEDs 2 are covering theentire perimeter of each square sheet 20 and are coupled to therespective edges of the sheet. According to one embodiment, lightextracting features 8 and 9 (not shown in FIG. 4 for clarity) may bedistributed over surfaces 11 and 21 with a constant density. Accordingto one embodiment, light extracting features 8 and 9 may be distributedover surfaces 11 and 21 with a variable density configured for enhanceduniformity of light output from stepped light guide 800.

According to one embodiment, LEDs 2 may include individually digitallyaddressable RGB LEDs. Such individually digitally addressable RGB LEDs 2may be selectively turned on and off or dimmed to illuminate selectareas of stepped light guide illumination system 900 in differentbrightness and/or color.

According to one embodiment, individual groups of LEDs 2 areindividually digitally addressable. For example, each group of LEDssurrounding individual sheet 20 may be controlled at once. According toan aspect, individual sheets 20 illuminated by the respective groups ofLEDs 2 may be configured and individually controlled as individual“pixels” within a large illuminated LED display. Such LED display mayincorporate hundreds or even thousands of sheets 20. For example, eachpixel including a single edge-lit sheet 20 may have a size of 1-2centimeters. Accordingly, an LED display having a size of 4 by 4 metersmay include from 40,000 to 160,000 such pixels.

According to one embodiment, each of sheets 20 may have a hexagonalshape. An exemplary arrangement of such hexagonal sheets 20 over surface11 is schematically shown in FIG. 5. Each of the side edges of hexagonalsheets 20 is provided with two LEDs 2 illuminating the respective edgesurface. It should be understood that such pattern of hexagonal sheets20 may extend continuously over surface 11 both longitudinally andlaterally (along the X and Y directions) to cover sufficiently largeareas. Channels 80 formed by hexagonal sheets 20 are arranged into threeparallel arrays that are crossed at an angle of 120° with respect to oneanother. Similarly to the embodiments discussed above, light extractionfeatures 8 and 9 may be provided on the respective surfaces 11 and 21 ofsheets 10 and 20. According to one embodiment, the patterns of lightextraction features 8 and 9 may be configured for a substantiallyuniform light output from the entire light emitting area of steppedlight guide illumination system 900.

FIG. 6 schematically shows an embodiment in which stepped light guideillumination system 900 is arranged into multiple independent sections905. Sections 905 are separated from each other by long and narrow airgaps 60 which longitudinally extend along the X and Y axes. Each section905 includes base light guiding sheet 10 and at least one light guidingsheet 20 attached to major surface 11 of sheet 10. LEDs 2 are providedaround the perimeter of each sheet 20 and are optically coupled to therespective edges of the sheet. Light control shields 62 may be providedwithin such air gaps between individual sections 905. Light controlshields 62 may be formed by strips of an opaque material positioned inair gaps 60 and oriented perpendicular to the XY plane. Light controlshields 62 may be configured to limit light passage from one section 905to another or even completely optically isolate sections 905 from eachother. One or more light control shields 62 may be mirrored for highspecular reflectance. One or more light control shields 62 may be madelight absorbing, e.g., formed by a black, light absorbing film material.

According to one embodiment, air gaps 60 may be minimized to a near-zerowidth. According to an alternative embodiment, sections 905 may bepositioned in an immediately adjacent position with respect to eachother, with no or very minimal air gaps 60. Additionally, sections 905may be attached to each other at the respective edges and may be furtheroptically coupled to each other with index matching or even fusedtogether to form a single, monolithic, LED-illuminated light guidingstructure, for example to allow for a generally unimpeded light passagefrom one section 905 to another.

FIG. 7 schematically shows an embodiment of stepped light guideillumination system 900 in a cross-section that is parallel to the YZplane. In this cross-section, the outline of base light guiding sheet 10is defined by opposing parallel surfaces 11 and 12 and by opposing edgesurfaces 13 and 14 than extend perpendicular to surfaces 11 and 12. Eachof sheets 20 attached and optically coupled to sheet 10 with indexmatching is defined by a pair of parallel opposing major surfaces 21 and22 and a pair of opposing edge surfaces 23 and 24. Base light guidingsheet 10 has a thickness T₁ and each of light guiding sheets 20 has athickness T₂.

In the illustrated example, T₁ is about equal to T₂. However, it isnoted that this inventions is not limited to this and may be applied tothe cases where T₁<T₂, T₁<<T₂, T₁>T₂, and T₁>>T₂. According to someembodiments, T₁≤T₂<1.5T₁, 1.5T₁≤T₂<2T₁, 2T₁≤T₂<3T₁, and 3T₁≤T₂<5T₁.According to alternative embodiments, T₂≤T₁<1.5T₂, 1.5T₂≤T₁<2T₂,2T₂≤T₁<3T₂, and 3T₂≤T₁≤5T₂.

LEDs 2 are positioned in a close proximity to the respective light inputedge surfaces 23 and 24. The size of each LED is preferably equal to orless than T₂. Each LED 2 is provided with a light collimating elementwhich is exemplified by a pair of reflectors 3 extending from a base ofthe LED at an angle of about 45% with respect to the base plane.Reflectors 3 are configured to intercept off-axis light rays emitted byLEDs 2 and direct such rays towards the respective edge surfaces 23 or24 thus enhancing the light coupling efficiency.

Each LED 2 is mounted to an individual heat spreading substrate 31 andis further provided with a finned heat sink 39 for enhanced heatdissipation. Each individual substrate 31 is mounted to a common heatspreading substrate 90. Substrate 90 is defined by opposing parallelsurfaces 92 and 94 and may include one or more metallic layers (e.g.,copper foil) for enhanced heat spreading and dissipation. Either one orboth substrates 90 and 31 may be exemplified by a Printed Circuit Board(PCB). According to one embodiment, the PCB of each substrate 31 mayinclude a metal core or can alternatively be attached to a metal sheet.

Surface 92 may be optionally mirrored for high specular reflectance.Alternatively, surface 92 may be provided with a diffuse reflectivecoating. According to some embodiments, it is preferred that thereflectance of surface 92 is at least 80%, more preferably at least 85%,even more preferably at least 90%, end still even more preferably atleast 95%. Major surfaces of substrates 31 that are facing light inputedges (edge surfaces 23 or 24) may optionally be mirrored or providedwith a high diffuse reflectance.

Referring further to FIG. 7, a high-reflectance sheet 41 of a highlyreflective material is provided between each surface 21 and substrate90. High-reflectance sheet 41 is positioned in a close proximity tosurface 21 and preferably in an immediately adjacent position to surface21. According to one embodiment, high-reflectance sheet 41 is made froma dielectric film material that has a hemispherical reflectance in thevisible spectrum of at least 90% and more preferably 95% or more. Thehigh reflectance of a specular type may be provided by metallization. Byway of example and not imitation, high-reflectance sheet 41 may beformed by a multi-layer optical film, such as Enhanced SpecularReflector (ESR) film commercially available from 3M. According to oneembodiment, high-reflectance sheet 41 has a diffuse reflectance of atleast 90-95% and preferably 96-99%.

Each exposed portion of surface 11 is provided with a high-reflectancesheet 40 that is positioned in a close proximity to surface 11 and morepreferably in an immediate adjacent position with respect to surface 11.In a non-limiting example, sheet 40 may be made from the same materialas sheet 41. According to one embodiment, sheet 40 is bonded to surface11 using an optically transmissive adhesive.

Edge surfaces 23 and 24 and the portions of surface 11 flanked bysurfaces 23 and 24 define rectangular channels 80 extending parallel tothe X axis. Substrate 90 encloses channels 80 and forms a plurality ofhollow chambers that extend parallel to the X axis and have a generallyrectangular cross-section. The size of such hollow chambers should besufficient to accommodate the respective strips of LEDs 2 with theassociated substrates 31 and heat sinks 39, as well as any associatedelectrical interconnects, as required. The hollow chambers may beventilated by a natural convection or forced convection using a fanpositioned at an entrance and/or exit of each channel.

Referring yet further to FIG. 7, stepped light guide illumination system900 further includes several optical management layers on a top side ofstepped light guide 800 (on the side of light output surface 12). Afirst optical management layer is exemplified by a light diffusing sheet42 which is laid on top of surface 12. It is preferred that there is atleast a thin layer of air provided between surface 12 and the lightreceiving surface of light diffusing sheet 42. To accommodate thisrequirement, light diffusing sheet 42 may be provided with surfacerelief features that prevent forming a continuous optical contact withsurface 12.

A second optical management layer is exemplified by a first prismaticfilm 44. Prismatic film 44 includes a plurality of parallel surfacemicroprisms that can deflect light rays emerging from light diffusingsheet 42. According to one embodiment, such surface microprisms areisosceles and are defined by planar surfaces that form a right anglewith respect to each other. Prismatic film 44 may be exemplified by aBrightness Enhancement Film (BEF) that is commonly employed in LCDdisplays. The BEF forming prismatic film 44 may be oriented such thatits linear surface microprisms are facing away from surface 12 andlongitudinally oriented parallel to the Y axis or at some angle (up to20° or up to 30°) with respect to the axis Y.

A third optical management layer is exemplified by a second prismaticfilm 46 having surface microprisms 47 oriented parallel to the X axis orat some angle (up to 20° or up to 30°) with respect to the X axis. Byway of example and not limitation, prismatic film 46 may be formed by asecond BEF that is oriented perpendicular or near-perpendicular to thefirst BEF (prismatic film 44). According to an alternativeimplementation, the orientations of the first and second BEFs may beswitched.

According to one embodiment, the first and second BEFs may be bonded toeach other at the tips of the bottom BEF. For example, a thin layer ofoptically clear adhesive may be provided on a bottom surface ofprismatic film 46 such that this adhesive bonds portions of theprismatic tips of prismatic film 44 to the bottom surface of prismaticfilm 46 when the films are pressed against each other.

According to one embodiment, the top BEF may be replaced with orcomplemented by an optically transmissive light diffusing film.According to one embodiment, such optically transmissive light diffusingfilm may include a two-dimensional array of micro lenses formed in asurface of the film that is facing away from stepped light guide 800.

Light extraction features 9 are exemplified by dome-shaped cavitiesformed in surface 21. Light extraction features 8 are exemplified bydome-shaped cavities formed in surface 11. The cavities may also haveprismatic shapes or any other shapes suitable for deflecting light. In anon-limiting example, the embodiment of FIG. 7 may be configured as abacklight unit and incorporated into an LCD display.

In operation, a light ray R4 emanated by individual LED 2 enters lightguiding sheet 20 through light input edge surface 23 and furtherpropagates in sheet 20 in response to optical transmission and TIR. RayR4 is then reflected from surface 21 and enters light guiding sheet 10.Since sheet 20 is optically coupled to sheet 10 with refractive indexmatching, ray R4 enters into sheet 10 without appreciable change inpropagation direction or energy loss at the optical interface formed bysurfaces 22 and 11. Ray R4 is further reflected from surface 12 by meansof TIR. Ray R4 then continues its propagation in light guiding sheet 10along the Y axis. It may be appreciated that ray R4 may subsequentlyencounter one of light extraction features 8 formed in surface 11 andmay be redirected towards light output surface 12 and extracted fromstepped light guide 800, as illustrated by a light ray R10.Alternatively, ray R4 may further re-enter the same or different sheet20 and encounter one of light extraction features 9 formed in surface21, in which case ray R4 may be similarly redirected towards lightoutput surface 12 and extracted from stepped light guide 800 towards anintended direction of light emission.

A different light ray R1 propagating in light guiding sheet 20 in awaveguide mode strikes one of light extracting features 9 where it isdeflected from its original propagation path and redirected towardslight output surface 12 so that it can exit from stepped light guide800, contributing to the overall light output from stepped light guideillumination system 900.

Light rays R2 and R6 illustrate the operation of stepped light guide 800in conjunction with the light management layers described above. Ray R2exiting from light output surface 12 strikes prismatic film 46 and isreflected back towards the light guide by one of microprisms 47. Ray R6exiting from light output surface 12 strikes prismatic film 44 and issimilarly back towards the light guide by the respective microprisms.Ray R6 is subsequently recycled by reflecting from high-reflectancesheet 41 and is again directed towards light output surface 12 so it canhave a second chance to exit from stepped light guide illuminationsystem 900.

It may be appreciated that the illustrated structure of stepped lightguide illumination system 900 in which one or more BEFs is disposed onthe light emitting side of light guide 800 and high reflectance sheets40 and 41 are disposed in the opposite side of the light guide may becapable of collimating light that is emitted from system 900.Accordingly, such structure may be advantageously selected forapplications where such collimation is required, e.g., for maximizingthe apparent brightness of a backlit LCD display.

Similarly, high-reflectance sheet 40 may be configured for recyclinglight that exits from sheet 10 into the hollow chambers formed bychannels 80, as schematically illustrated by a light ray R11. Surfacesof high-reflectance sheet 40 that are facing the hollow chambers may bemade optically absorptive. The light absorbing side of high-reflectancesheet 40 may be configured, for example, for blocking stray lightemanated by LEDs 2. The stray light may include light rays that are notcoupled to sheets 20, for example, due to a non-zero air gap betweenLEDs 2 and edge surfaces 23 and/or 24. The stray light may also includelight rays that are reflected from surfaces 23 and/or 24 by means ofFresnel reflection or light rays that exit from sheets 20 throughsurfaces 23 or 24. The light absorbing side of high-reflectance sheet 40may also be used for shielding light rays reflected from the interior ofthe hollow chambers (e.g., reflections from surface 92).

A light ray R8 illustrates the operation of light diffusing sheet 42.Ray R8 exiting from stepped light guide 800 is scattered by lightdiffusing sheet 42 and may further be redirected by prismatic films 44and 46, resulting in a diffuse fan of rays emitted from stepped lightguide illumination system 900.

FIG. 8 schematically illustrates an embodiment of stepped light guideillumination system 900 in which LEDs 2 are exemplified by side-emittingLEDs mounted directly on a common PCB 96 disposed below stepped lightguide 800. The term “side-emitting LED” is directed to mean an LED thatis packaged so that the LED's light emitting active layer or lightemitting aperture is perpendicular to the surface of a PCB or a heatspreading substrate used for mounting the LED. Conversely, the term“top-emitting LED” is directed to mean an LED that is packaged so thatthe LED's light emitting active layer or light emitting aperture isparallel to the respective PCB or heat spreading substrate and is lightemitting side is generally facing away from the PCB or heat spreadingsubstrate.

Side-emitting LEDs 2 are arranged such that their light emittingapertures 50 are aligned relatively to light input edge surfaces 23 and24, respectively. Light emitting apertures 50 are formed by therespective light emitting surfaces of LEDs 2. According to oneembodiment, LED each LED 2 includes a cavity 51 containing an LED chipor die 99. Cavity 51 may have reflective walls for directing and/orrecycling light. Cavity 51 may be conventionally filled with anencapsulation material, such as silicone. The encapsulation material mayfurther include luminescent or fluorescent materials, such as phosphors,for wavelength conversion. A size S₅₀ of light emitting aperture 50 ispreferably smaller than the thickness of sheet 20. According to oneembodiment, 0.3T₂<S₅₀<T₂. According to one embodiment, 0.5T₂<S₅₀<0.9T₂.According to one embodiment, S₅₀=½ (T₁+T₂). According to one embodiment,0.25 (T₁+T₂)<S₅₀<0.5T₂. According to one embodiment,0.3(T₁+T₂)<S₅₀<0.5T₂. According to one embodiment, 0.2(T₁+T₂)<S₅₀<0.5T₂.

Side-emitting LEDs 2 may be arranged in series and mounted to aself-adhesive strip. Such strip of LEDs 2 may be attached to a PCB or aheat spreading substrate below stepped light guide 800. Alternatively,the LED strips may be attached directly to a surface of stepped lightguide 800. According to one embodiment, high-reflectance sheet 40 isbonded to surface 11 using an optically clear adhesive, andself-adhesive strips of LEDs 2 are bonded to the exposed back surface ofsheet 40. When strips of side-emitting LEDs 2 are bonded directly tosurface 11, it is preferred than the bottom surface of each LED stripthat is used for attaching the strip to surface 11 has a sufficientlyhigh specular or diffuse reflectance. FIG. 8 further illustrates lightextraction features 8 and 9 that are formed by dots of a lightscattering material deposited to the respective surfaces 11 and 21.According to one embodiment, light extraction features 8 may be formedby a flexible or rigid substrate attached to surface 11. Such substratemay be configured as a strip of having high-reflectance, light-diffusingsurface facing surface 11 and an array of side-emitting LEDs 2 attachedto an opposite surface of the strip.

FIG. 9 schematically illustrates an embodiment of stepped light guideillumination system 900 in which LED 2 is exemplified by a top-emittingLED having multiple LED chips or die positioned within light-recyclingcavity 51. A layer 145 of an optically transmissive adhesive material orencapsulant is provided between light emitting aperture 50 and edgesurface 23. Such adhesive material or encapsulant may be configured toprovide refractive index matching between the light emitting surface ofLED 2 and light input surface 23 of sheet 20, thus eliminating the airgap.

Sheets 10 and 20 of the embodiment illustrated in FIG. 9 are separatedfrom each other by a relatively small distance. This distance ispreferably much less than the thicknesses of either one of sheets 10 and20. A layer 544 of a highly transparent adhesive material is providedbetween sheets 10 and 20. Layer 544 bonds together surfaces 11 and 22and provides good optical contact between such surfaces with refractiveindex matching. Adhesive layer 544 is applied only to a portion of theavailable contact area of sheets 10 and 20, which defines an opticalcontact section 52 and a gap section 53 where an air gap exists betweensheets 10 and 20.

An opaque light control sheet 43 is positioned within the air gap. Lightcontrol sheet 43 may be configured for rejecting stray light enteringfrom sheet 20 into sheet 10 in gap section 53, as illustrated by anexemplary ray R12. Light control sheet 43 may have both sides mirroredfor high specular reflectance. Alternatively, the side that is facingtowards sheet 10 may be reflective and the side that is facing sheet 20may be light absorbing or light scattering. The light coupling operationis further illustrated by an example of raytracing a light ray R11.

Ray R11 propagates from LED die 99 to sheet 20 without being trapped byTIR within cavity 51. According to one embodiment, layer 145 may beconfigured to prevent such light trapping in cavity 51 by means ofrefractive index matching and eliminating an air gap between LED 2 andsurface 23. Ray R11 further propagated from sheet 20 to sheet 10 withoutbeing trapped in sheet 20 by TIR due to the refractive index matchedoptical coupling between sheets 10 and 20.

Optical contact section 52 defines the area of optical contact betweensheets 10 and 20. The widths of optical contact section 52 and gapsection 53 along the Y axis may be selected based on various factors.For example, a width of gap section 53 (as measured along the Y axis)may be defined by a spacing distance between LEDs 2 along the X axis.For example, the gap width may be selected such that the individuallight beams emitted by LEDs 2 can overlap and/or superimpose on oneanother before reaching section 52 and/or the nearest light extractionfeatures 9. Accordingly, an improved homogeneity of light emissiontowards surface 12 may be obtained. Light extraction features 9 may beformed only in section 52 and may be generally avoided in section 53 forthe same reason. According to different embodiments, the width ofsection 53 along the Y axis is greater than 1.5 times, 2 times, 2.5times, and 3 times a spacing distance between LEDs 2 along the X axis.

The area of surface 11 covered with light extraction features 8 mayextend into section 53, as illustrated in FIG. 9. At the same time, thearea of surface 21 that corresponds to section 53 may be substantiallyfree from light extraction features 9. According to an alternativeembodiment, section 53 may include light extraction features 9 and besubstantially free from light extraction features 8.

According to one embodiment, referring further to FIG. 9, sheets 10 and20 may be separated from each other along the entire surfaces 11 and 22by a continuous air gap such that light can travel within each of thesheets considerable distances in a waveguide mode without appreciablecross-talk between the sheets. Alternatively, sheets 10 and 20 may bedisposed in physical contact with each other such that there is still nooptical contact or a very limited optical contact between surfaces 11and 22, e.g., due to the presence of a very thin (on a micrometer scale)air gap between the surfaces.

According to one embodiment, the material of layer 544 may have arefractive index that is considerably lower than that of either one orboth of sheets 10 and 20. Such low-refractive-index layer 544 can serveas an optical cladding layer that limits the optical cross-talk betweenthe sheets. For example, both of sheets 10 and 22 may be made frompolycarbonate (n≈1.58) and layer 544 may be formed by silicone withn≈1.41-1.43.

FIG. 10 schematically illustrates a top plan view of exemplary outlinesof optical contact section 52 and gap section 53 for an exemplary casewhere LEDs 2 are provided around the entire perimeter of individualsheet 20. FIG. 11 schematically illustrates exemplary outlines ofoptical contact section 52 and gap section 53 for an exemplary casewhere LEDs 2 are provided at only two opposing edges of sheet 20.

FIG. 12 schematically illustrates an embodiment of stepped light guideillumination system 900 in which light input edge surface 23 is inclinedat an angle with respect to a normal 220 to a prevalent plane of steppedlight guide 800. In the illustrated embodiment, the prevalent plane ofstepped light guide 800 may be defined as a plane that is parallel theXY plane. The optical axis of LED 2 is accordingly inclined at the sameangle with respect to the Y axis. Such configuration of light guidingsheet 20 may be advantageously selected, for example, to enhance theefficiency of light coupling into stepped light guide 800.

As further shown in FIG. 12, LED 2 may be further associated with alight collimating element 64 configured to collimate and direct lightemitted by the LED towards edge surface 23. A light ray R14schematically illustrates light reflection from high-reflectance sheet40 back to the body of sheet 10.

FIG. 13 schematically illustrates an embodiment of stepped light guideillumination system 900 in which sheet 20 has a tapered, wedge-shapedlight input section 66 which is configured for concentrating the lightbeam emitted by LED 2 and entering sheet 20. According to someembodiments, section 66 has a maximum thickness that is greater than thethickness of the rest of sheet 20, by at least 10%, 20%, 30%, 40% or50%. Section 66 may be particularly configured to accommodatelarger-size LEDs 2 while keeping the thickness of sheet 20 below acertain maximum desirable thickness. For example, size S₅₀ of lightemitting aperture 50 of LED 2 may be greater than thickness T₂ of sheet20 by at least 10%, 20%, 30%, 40% or 50%.

According to one embodiment, the light input edge of sheet 20 may beshaped in the form of a light-collimating optical element. An example ofsuch a collimator-shaped light input edge is shown, for example in FIG.8 of the '361 Publication. Referring further to FIG. 13, an opaque lightmanagement sheet 45 is provided below tapered section 66 to block straylight that may escape from this section of sheet 20. According todifferent implementations, light management sheet 45 may be reflectiveor light absorbing.

FIG. 14 schematically illustrates an exemplary two-dimensional patternof light extraction features/elements 8 and 9 in a rectangular zone 901.Rectangular zone 901 includes one section 54 which is symmetricallyflanked by two sections 55. According to one embodiment, section 54 ofFIG. 14 may correspond to a rectangular optical contact area ofindividual sheet 20 with sheet 10 and sections 55 may correspond toportions of sheet 10 that are not covered by sheet 20. In section 54,light extraction features 9 are distributed in staggered columnsextending parallel to the X axis. The density of the columns graduallyincreases from the respective light input areas towards the center ofsection 54 along the Y axis. In sections 55, light extraction features 8are arranged in staggered columns which density increases along the Yaxis towards a boundary of rectangular zone 901 (outwardly from therespective sheet 20).

FIG. 15 schematically illustrates a combined distribution pattern oflight extraction features 8 and 9 for a larger zone 902 of stepped lightguide 800 and shows multiple sections 54 alternating with sections 55. Aspatial density of light extraction features 8 and/or 9 may be definedas the number of respective light extraction features per unit area. Ineach of sections 54 and 55 of zone 902, the respective spatial densitiesof light extraction features increase towards the center of the section.Accordingly, the overall distribution pattern includes alternating bandshaving different spatial densities of light extraction features.Furthermore, the spatial density is variable within each band.

A relative surface area of light extraction features 8 and/or 9 at anyparticular location of surfaces 11 and/or 21 may be defined as a sum ofthe individual areas of the respective light extraction features withina selected sampling area divided by the total area value of the samplingarea. For example, a relative surface area of 0.5 correspond to one-halfof the sampling area being occupied by the light extraction features. Arelative surface area equal to one means that the light extractionfeatures occupy 100% of the sampling area, with no spaces. Depending onthe size and shape of individual light extraction features, the spatialdensity and relative surface area may be bounded by variousrelationships.

According to an aspect, FIG. 15 illustrates varying the spatial densityof light extraction features 8 and 9 (and, hence, varying the relativearea occupied by light extraction features 8 and 9) by varying thespacing between individual light extraction features. However, it shouldbe understood than the relative area may also be varied by varying thesize of individual features, even at a constant spatial density. Forexample, increasing an area of each light extraction feature 8 or 9 bytwo times within a particular sampling area will increase the relativearea of such feature within the sampling area by about two times.

FIG. 16 schematically illustrates an embodiment of stepped light guideillumination system 900 in which optical adhesive layer 544 extends overthe entire area of surface 11. The area covered by optical adhesivelayer 544 also includes portions surface 11 that are free from sheets 20and that define sections 55. A light diffusing sheet 48 is attached toan exposed area of optical adhesive layer 544 within channel 80. Lightdiffusing sheet 48 may be made from an opaque film-thickness materialthat has a high diffuse reflectance of preferably 85% or more, morepreferably 90% and even more preferably 95%. According to an aspect,sheet 48 represents an optically diffusing layer that is opticallycoupled to sheet 10 with refractive index matching. According to anaspect, the light diffusing layer formed by sheet 48 in section 55replaces discrete light extraction features 8 illustrated in thepreceding figures.

In operation, sheet 48 collects light rays that are incident onto therespective area of surface 11 and reflects such light rays withdiffusion, as schematically illustrated by an example of a light rayR30. Light diffusing sheet 48 may be configured to collect and diffuselyreflect nearly all light that is incident onto the area of surface 11disposed directly above sheet 48. Light extraction features 9 areexemplified in FIG. 16 by shallow micro cavities which spacing isprogressively decreased with a distance from LEDs 2.

LEDs 2 of FIG. 16 are mounted to PCB 96 and may be exemplified byside-emitting LEDs. Top surfaces of side-emitting LEDs 2 may also beglued to the backside of light diffusing sheet 48. The void betweenopposing LEDs 2 within channel 80 may be filled with an encapsulantmaterial, such as silicone. The entire channel 80 may be encapsulated,e.g., by filling substantially all available voids within the channelwith an encapsulant. According to different embodiments, the encapsulantmay be optically transmissive or white-colored, with high diffusereflectance.

Examples of side emitting LEDs that may be suitable for LEDs 2 includebut are not limited to Micro SIDELED product series commerciallyavailable from OSRAM (e.g., LW Y87C, CUW Y3SH.B1 and LW Y1SG models ofwhite LEDs or LB Y8SG model of blue LEDs) or models NS2W364G andNS2W266G of white side-emitting LEDs manufactured by Nichia. It isnoted, however, that the embodiments of stepped light guide illuminationsystem 900 described herein may also be adapted to other types and formfactors of side-emitting LEDs and may further be adapted to any othertypes and many configurations of LEDs, including square, round orrectangular top-emitting LEDs of various architectures.

Furthermore, non-LED light sources can be used in place of LEDs 2, suchas, for example, lasers, fluorescent lamps, incandescent lamps,gas-discharge lamps, and OLEDs. LEDs 2 may incorporate LED arrays orarrays of LED dies within a single package. Suitable examples of suchLEDs as well as related methods of LED coupling to light guides(waveguides) which can be utilized in many embodiments of the presentinvention are disclosed, for example, in the '666 Publication.

FIG. 17 schematically shows an embodiment of stepped light guideillumination system 900 which operation was modeled using opticalraytracing simulations, as described in the Example 1 below. Theillustrated embodiment includes sheet 10 (a top sheet) and sheet 20 (abottom sheet) attached to bottom surface 11 of sheet 10. Together,sheets 10 and 20 form exemplary stepped light guide 800.

Stepped light guide illumination system 900 of FIG. 17 includes two LEDs2 each having a rectangular light emitting aperture which dimensionsapproximate those of edge surfaces 23 and 24. LEDs 2 are positioned in aclose proximity to respective edge surfaces 23 and 24 so thatpractically all light emitted by the LEDs is injected into sheet 20.Sheet 10 has a length L_(TOP) along the Y axis and thickness T₁ alongthe Z axis. Sheet 20 has a length L_(BOT) along the Y axis and thicknessT₂ along the Z axis.

High-reflectance sheet 41 is positioned below surface 21 in a closeproximity to the surface (with a minimum air gap). A pair ofhigh-reflectance sheets 40 are positioned below the exposed portions 91and 92 of surface 11, also with a minimum air gap. Exposed portions 91and 92 of surface 11 are provided with light extraction features 8 whichare formed by dots of highly reflective, diffuse material deposited tothe surface in predetermined locations. Light extraction features 8 andare distributed over surface portions 91 and 92 with a variable spatialdensity determined from raytracing. Light extraction features 9 areformed on surface 21 by dots the same material as light extractionfeatures 8. However, light extraction features 9 have a generallydifferent spatial density (compared to features 8) which is alsodetermined from raytracing. Opposing edge surfaces 13 and 14 of top basesheet 10 are mirrored with specular reflectance close to 100%. A widthof sheets 10 and 20 along the X axis is arbitrarily selected to beapproximately equal to L_(TOP).

It may be appreciated that the illustrated configuration may berepresentative of stand-alone system 900 including just single sheet 10and single sheet 20 attached to surface 11. On the other hand, FIG. 17may also represent a section or a “tile” being a part of larger system900 which includes a number of such sections or tiles. The tiles may beoptically isolated from each other at edge surfaces 13 and 14.Alternatively, multiple tiles may be contiguous and optically connectedto each other at edge surfaces 13 and 14. Accordingly, intensityprofiles along the Y axis obtained for the illustrated stand-aloneconfiguration of system 900 with simulated mirrored edge surfaces 13 and14 may also be representative of intensity profiles of configurations ofsystem 900 where edge surfaces 13 and 14 are not mirrored but used toconnect multiple tiles of the same design or where edge surfaces 13 and14 merely represent imaginary boundaries between contiguous sections ofthe same, continuous stepped light guide 800. A reference line 270represents an axis of symmetry of the exemplary design depicted in FIG.17.

Example 1

Stepped light guide illumination system 900 of FIG. 17 was modeled usingraytracing with the following parameters: L_(TOP)=100 mm; L_(BOT)=80 mm;the width of sections 91 and 92 being 10 mm each, T₁=T₂=5 mm; sheets 40and 41 being mirrored and having a specular reflectance of 90%, thereflectance of surfaces 13 and 14 being 100%; the material of sheets 10and 20 being PMMA (acrylic) with n=1.49; the height of light emittingaperture of LEDs 2 being 5 mm (same as the thickness of sheet 20). Aregistration surface 272 was positioned in a close proximity (about 0.1mm) from surface 12 to register all rays exiting from the surface. Thelight extraction features 8 and 9 were modeled as 0.25 mm square surfaceareas having a 100% diffuse reflectance and producing a lambertian raypattern upon reflecting an incident light beam. Sheets 10 and 20 weremated together with an ideal optical contact.

FIG. 19 shows a calculated relative luminance profile (shown as a curve1030) of light output surface 12 along the Y axis (see FIG. 17). Therelative uniformity of surface luminance was obtained by iterativelyadjusting the relative surface area of light extraction features 8 and 9at various locations of surfaces 21 and 11, respectively.

FIG. 18 shows a calculated distribution of the relative surface area oflight extraction features 8 and 9 that produced the luminance profile ofFIG. 19. A curve 1020 represents the distribution of a relative surfacearea of light extraction features 9 over surface 21 (in section 54).Curves 1021 and 1022 represent the distributions of a relative surfacearea of light extraction features 8 over surface 11 in sections 91 and92 (also sections section 55).

As it can be seen from FIG. 18, the combined distribution of relativesurface area of light extraction features 8 and 9 exhibits a steppedchange in the density of light extraction features at two transitionareas. These transition areas correspond to the locations of steppedchanges in the thickness of stepped light guide 800 (the locations ofedge surfaces 23 and 24).

The respective stepped changes in the density of light extractionfeatures may be characterized by a magnitude M₁₀₂₀. Magnitude M₁₀₂₀ maydepend on several factors, including, for example, the differencebetween thicknesses T₁ and T₂, the overall thickness of stepped lightguide 800, its length and the relative length of the side sections ofsheet 20. In the illustrated case, magnitude M₁₀₂₀ is about 0.3 as therelative area of light extraction features undergoes stepped changesfrom about 0.3 to about 0.6 at the transition location from sheet 20(section 54) to sheet 10 (sections 55). In other words, the magnitude ofthe stepped change in the relative area of light extraction features isof the order of 2.

End of Example 1

Accordingly, as shown by the Example 1, stepped light guide illuminationsystem 900 can be designed for a substantially uniform light output fromthe entire area of surface 12 by appropriately selecting thedistribution density of light extraction features 8 and/or 9 and withoutvarying other optical parameters of the system. According to oneembodiment, the luminance of surface 12 can be made uniform within 25%from an average luminance of the surface, more preferably within 20%,even more preferably within 15% and still even more preferably within10%.

According to an aspect of the graph of FIG. 18, magnitude M₁₀₂₀ mayrepresent a difference between a minimum relative surface areaRA_(MIN_9) of light extraction features 9 and a minimum relative surfacearea RA_(MIN_8) of light extraction features 8. According to differentembodiments, a ratio RA_(MIN_8)/RA_(MIN_9) may be at least 1.3, at least1.5, at least 2, and at least 3 or more.

According to an aspect, a ratio between an average or total relativesurface area RA_(AVG_8) of light extraction features 8 and an average ortotal relative surface area RA_(AVG_9) may be greater than a unity.According to different embodiments, a ratio RA_(AVG_8)/RA_(AVG_9) may beat least 1.1 or more, at least 1.2 or more, at least 1.3 or more, atleast 1.4 or more, and at least 1.5 or more.

According to one embodiment, the relative surface area or density oflight extraction features 8 may be selected to minimize light transferthrough sections 55 and cross-talk between adjacent sections 54. Forexample, a sufficiently high density for light extraction features 8 maybe selected to intercept more than 50%, more than 75%, more than 85%,and more than 90% of light rays entering the respective section 55 fromadjacent section 54.

It may be appreciated that a relative high density of light extractionfeatures 8 (e.g., compared to the density of light extraction features 9near the light input edges of sheets 20) may allow for opticallyisolating sheets 20 from each other. For example, in embodiments ofstepped light guide illumination system 900 employing multiple sheets20, the density of light extraction features 8 may be selected to allowless than 10% of light to travel from one sheet 20 to adjacent sheet 20through section 55 separating such sheets. Greater densities of lightextraction features 8 may also be advantageously selected for the caseswhere the widths of sections 55 (as measured along the Y axis, forexample) are relatively small, e.g., to additionally enhance the rate oflight extraction in such sections 55.

FIG. 20 schematically depicts an embodiment of stepped light guideillumination system 900 in which adjacent sheets 20 are positionedfairly close to each other, providing a relatively narrow spacing forpositioning LEDs 2 (narrow section 55). A series of LEDs 2 illuminatingboth sheets 20 are arranged into a single linear array or strip of LEDs.Some LEDs 2 are facing towards edge surface 24 and some LEDs 2 arefacing towards edge surface 23. LEDs 2 that are facing edge surface 24are alternating with LEDs 2 that are facing edge surface 23 to form analternating pattern of light sources illuminating both sheets 20 andarranged into a single row.

FIG. 21 schematically depicts an embodiment of stepped light guideillumination system 900 which is similar to that of FIG. 20 except thatlight input edges of sheets 20 have surface corrugations 330 formed inrespective edge surfaces 23 and 24. Such surface corrugations 330 may beconfigured to disperse light entering sheets 20 in the XY plane (asschematically illustrated by light paths of individual rays) and improvelight mixing within sheets 20. As illustrated in FIG. 21, each surfacecorrugation 330 may have a symmetrical configuration relatively the Yaxis in a cross section parallel to the XY plane. According to differentembodiments, each surface corrugations 330 may have a symmetricalprismatic shape, a trapezoidal shape, a circular/hemispherical shape, ora partial circular shape. According to one embodiment, surfacecorrugations 330 are formed by an array of rounded ridges or lenticularlenses longitudinally extending parallel to the Z axis.

Surface corrugations 330 may be formed in any edge surface of sheets 10and/or 20 that is illuminated by an external light source, e.g., LEDs 2.For example, referring to FIG. 1 and FIG. 2, surface corrugations 330may be formed in each or in any combination of surfaces 13, 14, 15, 16,23, 24, 25, and 26.

FIG. 22 schematically depicts an embodiment of stepped light guideillumination system 900 in which sheet 10 and 20 are combined togetherto form an integral, single-piece, stepped light guiding structure oflight guide 800 operating according to the same principles discussedabove. Such monolithic structure may be formed from a solid dielectricmaterial as a single piece. For example, it may be formed by means ofinjection molding or compression molding. In a further example, multiplesheets 20 may be overmolded on a common substrate formed by sheet 10. Ina yet further example, sheets 10 and 20 may be formed from differentmaterials and permanently bonded or fused together to form an integral,monolithic light guiding optical structure.

According to one embodiment, channels 85 of FIG. 22 may be formed bymachining a blank, single-piece sheet of glass or plastic material. Forexample, a method of making stepped light guide 800 may includeproviding a planar sheet of optically clear PMMA and machining aparallel array of channels 85 using a rotary milling bit. The millingbut may have diamond-tipped edges and may be shaped to produce arectangular groove or channel in a single pass or multiple passes mymoving it through the material of PMMA sheet along the X axis. Similarlya second, perpendicular, array of channels 85 may be machined using thesame bit by moving it in a perpendicular direction (along the Y axis).

Referring to FIG. 22, monolithic stepped light guide 800 is defined by afront light emitting surface 212, a rear surface 214 and a plurality ofparallel channels 85 having rectangular cross-sections. Channels 85 maybe arranged in a single parallel array or two parallel arrays crossed atan angle with respect to each other, such as, for example, two parallelarrays that are orthogonal to each other. Channels 85 may be configuredand distributed over the area of stepped light guide 800 in a similarfashion as channels 80 described in the preceding embodiments.

Each channel 85 defines a pair of side surfaces 82 and 83 and a basesurface 213. LEDs 2 are coupled to side surfaces 82 and 83. Lightextraction features 8 are formed in base surfaces 213 and lightextraction features 9 are formed in portions of rear surface 214 betweenchannels 85. The portions of stepped light guide 800 having a greaterthickness define sections 54 and the portions having a lower thicknessdefine sections 55. Channels 85 may be optionally encapsulated, e.g.,partially or completely filled with silicone to form a continuoussheet-form illumination structure of a constant or near-constantthickness with LEDs 2 embedded into such structure. A layer of adhesivemay be provided on rear surface 214 of stepped light guide 800 tofacilitate attaching the light guide it to various surfaces. Accordingto one embodiment, a sheet of highly reflective, specular material,e.g., 3M's ESR film, may be provided on the side of surface 214 ofstepped light guide 800 and configured to reflect substantially alllight emerging from surface 214. The highly reflective sheet may bebonded to surface 214 using an optically clear adhesive. According to analternative embodiment, the highly reflective sheet may be of a diffusetype in which case it should be positioned in a close proximity tosurface 214 or disposed in physical contact with surface 214 providedthat the optical contact between the highly reflective sheet and surface214 is basically prevented or at least minimized.

FIG. 23 schematically depicts a partial section view an embodiment ofstepped light guide illumination system 900 in which top light emittingsurface 12 is provided with surface microstructures. Suchmicrostructures are exemplified by an array of parallel rounded ridgesdefining lenticular or cylindrical linear lenses 208. FIG. 24schematically depicts a perspective view of an embodiment of steppedlight guide illumination system 900 which has a similar basicarrangement as the embodiment depicted in a cross section in FIG. 23.

According to one embodiment, the rounded ridges forming lenses 208 maybe formed directly in surface 12 such that sheet 10 has a monolithic,gapless structure formed from a single piece of an opticallytransmissive material. In other words, lenses 208 may be made anintegral part of the respective light guide (sheet 10) and maycumulatively form a structured light guiding surface.

According to one embodiment, the rounded ridges forming lenses 208 maybe formed in a separate sheet of a highly optically transmissivepolymeric material (e.g., PMMA) which can be bonded to sheet 10 to forma monolithic, integral sheet having structured light-guiding surface 12.A thin layer of an optically clear adhesive or encapsulant may be usedfor bonding such lens-array sheet to base sheet 10. According to oneembodiment, the thin layer of an optically clear adhesive or encapsulantmay be refractive index matched with the material of sheet 10 and/orlens array. According to one embodiment, the thin layer of an opticallyclear adhesive or encapsulant may have a lower refractive index than thematerial of sheet 10 and may form a cladding layer. According to oneembodiment, the material of the lens array may have a lower refractiveindex than the material of sheet 10. According to one embodiment, thelens array may be separated from base sheet 10 by a thin layer of air orhighly transparent material such that there is no optical contact orlimited optical contact between the lens array and base sheet 10.According to one embodiment, the lens array may be disposed in physicalcontact with base sheet 10 such that there is no optical contact orlimited optical contact between the lens array and base sheet 10.

Lenticular lenses 208 extend parallel to each other and parallel to theY axis such that each of lenticular lenses 208 extends perpendicular toedge surfaces 13 and 14. Lenses 208 may be separated by spacing areas215. According to one embodiment, spacing areas 215 may be exemplifiedby smooth and planar portions of surface 12 between lenses 208. Therespective smooth and planar portions of surface 212 may extendsubstantially parallel to the XY plane. According to one embodiment,spacing areas 215 may include other surface structures such as roundedridges or linear cylindrical lenses each having a smaller width thanlenses 208 and extending parallel to lenses 208. According to oneembodiment, spacing areas 215 may include linear microprisms orprismatic corrugations formed in surface 12 and extending parallel tolenses 208.

According to one embodiment, the transverse width of spacing areas 215is approximately equal to the transverse width of lenses 208 (asmeasured along the X axis). According to one embodiment, the width ofspacing areas 215 is less than the width of lenses 208 by about twotimes or more. According to one embodiment, the width of spacing areas215 is greater than the width of lenses 208 by about two times or more.According to one embodiment, the width of spacing areas 215 is much lessthan the width of lenses 208. The width of spacing areas 215 can also bemade virtually zero. In other words, lenses 208 can be distributedacross surface 12 with a high packing density and may also be contactingeach other or positioned in an immediate proximity to each other.

Referring to FIG. 23 and FIG. 24, LEDs 2′ incorporated into a firstarray are coupled to edge surfaces 23 and 24. LEDs 2 incorporated into asecond array are coupled to edge surfaces 25 and 26. An additional,third array of LEDs 202 is provided. LEDs 202 are positioned adjacent toedge surfaces 13 and optically coupled to base sheet 10. Similarly, thethird array of LEDs 202 may include additional LEDs 202 that are coupledto opposite edge surface 14 of sheet 10.

Linear cylindrical lenses 208 extend along the light propagation pathwith respect to the light injected into sheet 10 through edge surfaces13 and/or 14. Lenses 208 can be made integral to sheet 10 and can beconfigured reflect the light guided within sheet 10 by means of TIR.Lenses 208 should normally have smooth surfaces polished to a high glossand preferably to optical-quality surface finish. An root mean squaresurface roughness parameter of lenses 208 should preferably be less than20 nanometers and more preferably 10 nanometers or less.

Accordingly, in operation, light emitted by LEDs 2 and/or LEDs 202 canbounce from lenses 208 and opposing surfaces 11 and 21 of stepped lightguide 800 and can be guided along the Y axis until it is extracted byrespective light extracting features 8 and/or 9. Suitable configurationsof lenses 208 as well as related methods of forming and using suchlenses with planar light guides (waveguides) are disclosed, for example,in the '826 and '007 patents.

According to one embodiment, lenses 208 may be configured to assist inmixing light emitted by LEDs 202 within stepped light guide 800. In thiscase, the focal length of the lenses may be selected to be significantlyshorter than the thickness of sheet 10 or at least shorter than thecombined thickness of sheets 10 and 20. According to differentembodiments, the focal distance may be less or equal to one third of thethickness of sheet 10, less or equal to one-half of the thickness ofsheet 10, or less or equal to the thickness of sheet 10. At the sametime, it may be preferred that the focal distance is greater than onetenth of the thickness of sheet 10.

According to some embodiments, lenses 208 may be configured to assist insmoothing out the irregularities of the luminance of surface 12 that maybe associated with individual LEDs 202, patterns of discrete lightextraction features 8 and/or 9, or boundaries of sheets 20. Thecriticality of using lenses 208 and configuring them to maximize thelight diffusion effect may be particularly consistent with theembodiments of system 900 where the light extraction patterns, thetransitions between sheets 20 or the emission from individual LEDs 2,2′, 2′″, 2″″, or 2′″″ would otherwise be perceptible by a human observer(e.g., when system 900 is used within an lighting fixture or a backlightunit of an electronic display, according to at least some embodiments ofthe present invention).

According to one embodiment, the shape of lenses 208 may be selected toprovide a prescribed collimation of the emitted beam. Various examplesof light collimating configurations of light guides employing linearcylindrical lenses or point-focus lenses and consistent with theembodiments described herein are disclosed, for example, in the '007 and'826 patents. By way of a non-limiting example, FIG. 23A of the '007patent illustrates light distribution and collimation using lenticular(linear cylindrical) lenses that extend parallel to a light input edgeof a planar light guide. In a further non-limiting example, FIG. 26 ofthe '826 patent illustrate light distribution and collimation usinglenticular lenses that extend perpendicular to a light input edge of aplanar light guide and are formed directly in a light-emitting surfaceof the light guide. The respective teachings may be applied, withoutlimitations, for configuring lenses 208 formed in light guiding basesheet 10.

The degree of light collimation and/or mixing achieved using lenses 208may be controlled, for example, by a radius of curvature R of lenses 208(in a transverse cross section that is parallel to the XZ axis).Additionally, it can be controlled by the distance between lenses 208and light extraction features 8 and/or 9. Yet additionally, it can becontrolled by the relative sizes of lenses 208 and light extractionfeatures 8 and/or 9 (e.g., by a ratio between a transversal width oflens 208 and the size of light extraction feature 8 or 9). According toone embodiment, radius of curvature R of at least a substantial portionof the surface of each lens 208 may be defined by a lens maker'sequation discussed in the '826 patent.

According to one embodiment, radius of curvature R of at least asubstantial portion of the surface of each lens 208 may be bounded bythe following relationship:

${{TC} \approx \frac{nR}{n - 1}},$

where n is the index of refraction of the material of sheet 10 and or20, and TC is a dimensional parameter associated with the thickness ofeither one or both sheets 10 and 20.

According to one embodiment, TC is about equal to the thickness of sheet10. According to one embodiment, TC is about equal to the thickness ofsheet 20. According to one embodiment, TC is about equal to the combinedthickness of sheets 10 and 20 or the total thickness of stepped lightguide 800. According to one embodiment, TC is approximately equal to 0.5times the thickness of sheet 10. According to one embodiment, TC isapproximately equal to 0.3 times the thickness of sheet 10. According toone embodiment, TC is greater than 0.1 times the thickness of sheet 10and less than one-half the thickness of sheet 10. According to oneembodiment, TC is greater than 0.1 times the combined thickness ofsheets 10 and 20 and less than one-half the combined thickness of sheets10 and 20.

FIG. 25 schematically illustrates an embodiment of stepped light guideillumination system 900 which includes a parallel array of opticalcouplers 88 that are configured to assist in coupling light emitted byLEDs 202 into light guide 800.

Optical couplers 88 are exemplified by tapered ridges that extendparallel to the Y axis (perpendicular to edge surface 13). Such ridgesmay be formed from the same material as sheet 10. For example, they canbe molded as three-dimensional surface features directly in surface 212at the time of making sheet 10. All of the ridges can be formed at onceand together with lenses 208. Alternatively, the ridges may also beovermolded on top of surface 212 and can be made from a differentmaterial that has high optical transmittance, e.g., optical-grade PMMA(acrylic), polycarbonate or low-iron glass. In a further alternative,optical couplers 88 may be formed separately from sheet 10 and thenbonded to surface 12, e.g., using a UV-curable optical adhesive.

Each optical coupler 88 has a light input face 89 that extendsperpendicular to surface 12 and is disposed flush (i.e., in the sameplane or substantially the same plane) with edge surface 13. Surface 12has a light input portion that is smooth and substantially free fromlenses 208 and a light emitting portion that is microstructured andincludes lenticular lenses 208. Optical couplers 88 are formed in thelight input portion of surface 12. According to one embodiment, opticalcouplers 88 may be formed in the portions of surface 12 that have lenses208.

According to a preferred embodiment, sheet 10 has a thickness that islower than a height of each LED 202 (where the height is measured alongthe Z axis). Furthermore, the thickness of sheet 10 is preferable equalto or less than the size of a light emitting aperture of LEDs 202 (asmeasured along the Z axis). Each optical coupler 88 has a maximum height(measured along the Z axis at light input face 89) sufficient toaccommodate the size of the light emitting aperture of an individual LED202 measured along the Z axis. According to one embodiment, the sum ofthe thickness of sheet 10 and the maximum height of optical coupler 88is at least equal to or greater than the size of the light emittingaperture of LEDs 202. According to different embodiments, the sum of thethickness of sheet 10 and the maximum height of optical coupler 88exceeds the size of the light emitting aperture of LEDs 202 along the Zaxis by at least 10%, at least 20%, and at least 30%, but no more thanby 50%.

Each LED 202 is disposed in registration or alignment with respect to atleast one optical coupler 88 and is positioned in an immediate proximityto edge surface 13 and light input face 89 such that to minimize lightspillage. According to one embodiment, multiple LEDs 202 may be coupledto a single optical coupler 88.

The spacing between side edges of individual optical couplers 88 may beselected such that the optical cross-talk between adjacent opticalcouplers 88 is minimized. In other words, the spacing should besufficient to allow light that exits from one coupler 88 to propagatewithin sheet 10 away from edge surface 13 without entering into anadjacent coupler 88. The spacing should also preferably be wide enoughto prevent a cone of light emitted by an individual LED 202 that iscoupled to a particular optical coupler 88 to encompass adjacent opticalcouplers 88. According to one embodiment, a minimum spacing betweenoptical couplers 88 is equal to or greater than thickness T₁ of sheet10. According to some embodiments, such minimum spacing is equal to orgreater than T₁ times 1.5, equal to or greater than T₁ times 2, equal toor greater than T₁ times 2.5, and equal to or greater than T₁ times 3.

In operation, a light ray R55 illustrates an exemplary light path instepped light guide illumination system 900. Light ray R55 emitted byone of LEDs 202 enters into optical coupler 88 that is aligned with therespective LED 202. Light ray R55 enters into the optical couplerthrough face 89.

Light ray R55 further propagates within the space defined by the body ofoptical coupler 88 and a portion of sheet 10 below the couplerundergoing multiple bounces from the walls of optical coupler 88 andsurface 11 until it exits from this space and further propagates withina portion of the body of sheet 10 that is free from optical couplers 88.

Light ray R55 is further guided within sheet 10 in response to opticaltransmission and TIR. On its path, light ray R55 may encounter one ormore sheets 20 and can similarly propagate within such sheets inresponse to optical transmission and TIR. TIR may occur at any or all ofsurfaces 11, 12, and 21. TIR may also occur at edges of stepped lightguide 800.

Both the spacing areas 215 of surface 12 and lenses 208 formed in suchsurface are preferably configured to reflect light by means of TIR.Light ray R55 further encounters one of light extraction features 8 or 9(not shown for clarity) and is emitted from surface 12 while propagatingthrough one of lenticular lens 208. Lenticular lens 208 may additionallybend the propagation path of light ray R55 and can provide, for example,enhanced light diffusion or collimation.

Exemplary configurations of highly elongated optical couplers 88 andlight-guiding sheet 10 employing such optical couplers, as well as theprinciples of LED coupling to such couplers and light guiding sheet, caninclude those of face-lit waveguide illumination systems, for example,as described in the '666 Publication.

FIG. 26 schematically illustrates an embodiment of stepped light guideillumination system 900 which is similar to that of FIG. 25 except thatLEDs 202 and optical couplers 88 have different configurations,dimensions, and arrangement, and also provide for a somewhat differentmode of light coupling.

Referring to FIG. 26, each LED 202 has a highly elongated configuration,with its length along the X axis being much greater than the other twodimensions (e.g., by 2 times, 3 times, 3.5 times or 4 times or more).Accordingly, each optical coupler 88 is shaped in the form of a widewedge such that its light input face 89 extends along the same or agreater distance along the X axis as the length of individual LED 202. Atransverse width W₈₈ of each optical coupler 88 (as measured along the Yaxis) is much shorter than its length L₈₈ along the X axis. According todifferent embodiments, L₈₈ can be greater than transverse width W₈₈ by 2times, 3 times, 4 times, 5 times, and 6 times or more. According to oneembodiment, L₈₈ can be greater than 2 times W₈₈ and less or equal to 10times W₈₈.

Transverse width W₈₈ can be further constrained by a requirement tominimize secondary interactions of light rays with a top wall of opticalcoupler 88 and thus minimize premature light leakage from light guide800. The criticality of appropriate dimensioning of optical couplers inview of minimizing light spillage is disclosed, for example, in par.[0097]-[0104] of the '361 Publication. A critical width W_(max) ofoptical coupler 88 (as measured along the Y axis) may be defined by thefollowing expression:

${W_{m\; a\; x} = \frac{2d}{\sqrt{n^{2} - 1}}},$

where d is the thickness of sheet 10 and n is the refractive index ofthe material of sheet 10. For example, for polycarbonate (n=1.58,W_(max)=1.63 times d), and for PMMA (n=1.49, W_(max)=1.81 times d).

According to several embodiments, the transversal width W₈₈ of eachoptical coupler 88 (along the Y axis) can be approximately equal toW_(max), less than 1.5 times W_(max), less than 1.3 times W_(max), lessthan 0.8 times W_(max), and less than 0.5 times W_(max). According to atleast one embodiment, the transverse width of each optical coupler 88may be further constrained to be greater than 0.1 times W_(max) and morepreferably greater than 0.3 times W_(max). According to at least oneembodiment, the transverse width of each optical coupler 88 may begreater or equal to 0.75 times W_(max) and less than or equal to 1.25times W_(max).

According to preferred embodiments, a taper angle of optical coupler 88in the YZ plane should not exceed 15 degrees. The taper angle may bedefined at a dihedral angle between the top surface of optical coupler88 and a bottom surface of the coupler (or surface 12). Suitableexemplary ranges for the taper angle may include 3 to 6 degrees, 7 to 10degrees, and 10 to 15 degrees. According to different exemplaryembodiments, the taper angle may be 6 degrees, 8 degrees, 10 degrees,and 12 degrees. According to one embodiment, each optical coupler 88 mayfurther have a taper in the orthogonal XY plane.

As illustrated in FIG. 27, schematically depicting a cross-section of alight input portion of the embodiment shown in FIG. 26, a height T₈₈ ofwedge-shaped optical coupler 88 (as measured along the Z axis) isselected to accommodate a height H₂₀₂ of LED 202 (as measured along thesame axis), such that the combined thickness of the respective lightreceiving end of sheet 10 is at least equal to or preferably greaterthan height H₂₀₂. According to one embodiment, height H₂₀₂ may refer toa total height of the respective LED package. According to oneembodiment, height H₂₀₂ may refer to the total size of a light emittingaperture of the respective LED package (as measured along the Z axis).

FIG. 28 schematically illustrates an embodiment in which wedge-shapedoptical coupler 88 is formed by a tapered light input end of sheet 10where the thickness of sheet 10 at edge surface 13 is greater thannominal thickness T₁ of the sheet. Optical coupler 88 of FIG. 28 formsan integral part of sheet 10 such that light input edge surface 13 isalso light input face 89 of optical coupler 88.

Optical couplers 88 may be formed in either one of surfaces 11 and 12.Optical couplers 88 may also be formed in both surfaces 11 and 12.According to one embodiment, each optical coupler 88 may have asymmetrical tapered configuration (in a cross-section parallel to the YZplane), forming identical opposing protrusions in surfaces 11 and 12,for example, as illustrated in FIG. 29 and FIG. 30. The above describedlimitations regarding the dimensioning and positioning of asymmetrical(referring to a cross-section parallel to the YZ plane) optical couplers88 may be equally applied to the dimensioning and positioning of suchsymmetrical configurations of the couplers (referring to the samecross-section).

A total maximum thickness T_(max) of sheet 10 at the light input edge(i.e., at light input face 89 of optical coupler 88) may be defined asthe sum of nominal thickness T₁ of sheet 10 and the combined thicknessadded by optical coupler(s) 88. For example, T_(max)=T₁+T₈₈ for theembodiment of FIG. 27 and T_(max)=T₁+2T₈₈ for the embodiment of FIG. 30.It may be appreciated that many applications may require maximizinglight coupling efficiency. It can be shown that the light couplingefficiency may be maximized when certain relationships between thegeometrical parameters of sheet 10 and optical couplers 88 are met.

According to one embodiment, optical coupler is formed in only one ofsurfaces 11 and 12, T₁ is about 0.75 times T_(max), and T₈₈ is about0.25 times T_(max). According to one embodiment, optical coupler 88 isformed in both surfaces 11 and 11 (e.g., as illustrated in FIG. 30), T₁is about 0.75 times T_(max), and T₈₈ is about 0.125 times T_(max).According to one embodiment, T₈₈ is equal or greater than 0.25 timesT_(max) and equal to or less than 0.5 times T_(max). According to anaspect of the embodiments illustrated in FIG. 27, FIG. 29 and FIG. 30,the entire tapered portion of sheet 10 configured for light input fromindividual LED 202 or a compact cluster LEDs 202 of may be regarded asan individual optical coupler 88 and the above-described limitations maybe applied to configuring and dimensioning such tapered light inputportion of sheet 10.

According to some embodiments, each sheet 20 may be configured toinclude optical couplers on one, two, three or all four of its edges.Such optical couplers may have the same or similar configurations,dimensions and operation described above in reference to opticalcouplers 88 and edge surface 13 of sheet 10. According to oneembodiment, both sheets 10 and 20 may be provided with optical couplers88 on edges that are illuminated with LEDs.

According to one embodiment, stepped light guide 800 can be made fromPMMA (acrylic) and the outside medium can be air. Other suitablematerials for making stepped light guide 800 include but are not limitedto polycarbonate, glass, polystyrene, silicones, and any other soliddielectric materials that provide sufficient optical transmittance forguiding light along a prevalent plane of the stepped light guide.

According to one embodiment, linear cylindrical lenses 208 may bereplaced by parallel ridges formed in surface 212 and having any othersuitable shape. The parallel ridges may be rounded, e.g., lenticularlens-like. The parallel ridges may also be prismatic, e.g., formed by aparallel array of triangular microprisms. Similarly to lenses 208, suchparallel ridges may extend perpendicular to edge surfaces 13 and 14 orperpendicular to edge surfaces 15 and 16, and may be configured toreflect/guide light by means or TIR away from the respective edges.

FIG. 31 schematically depicts a portion of a light input edge of sheet10 where an array of edge surface corrugations 330 is formed in edgesurface 13 and an array of surface corrugations 331 is formed in lightinput face 89 of each optical coupler 88. Surface corrugations 330 and331 longitudinally extend parallel to the Z axis and are configured todisperse light entering into sheet 10 in the XY plane such that thecones of light from individual LEDs 202 can be spread over a widerangular range in that plane compared to the case of planar edge surface13 and light input faces 89.

According to one embodiment, all surface corrugations 330 and 331 mayhave the same transversal shape in a cross-section (in a plane parallelto the XY plane) and can have the same spacing, although their lengthsalong the Z axis can be different (due to the different thickness ofsheet 10 and couplers 88). According to one embodiment, surfacecorrugations 330 and 331 can have different cross-sectional shapesand/or spacing.

According to one embodiment, stepped light guide illumination system 900may be made sufficiently thin to enable easy flexing and conforming tovarious prescribed shapes. Flexible light guide illumination system 900may be formed by flexible stepped light guide 800 and a flexible PCB(e.g., PCB 96) carrying a two-dimensional array of LEDs 2. Flexiblestepped light guide 800 may be formed by sufficiently thin and flexiblesheets 10 and 20 so that the overall thickness of stepped light guide800 is about equal to or below 3 mm, more preferably equal to or below 2mm, more preferably equal to or below 1.5 mm, and even more preferablyequal to or below 1 mm. This may be possible, for example, with the useof low-profile side-emitting LEDs having light emitting aperturesbetween 0.1 mm and 0.6 mm in size (at least in a dimension thancorresponds to the Z axis in the preceding Figures) and thicknesses ofsheets 10 and 20 in the 0.1-1.5 mm range.

According to one embodiment, flexible stepped light guide 800 isconfigured or adapted for being retained in a bent or curvedconfiguration (e.g., form a backlight unit of a curved LCD display).According to one embodiment, such light guide 800 may include lenses 208that extend perpendicular to a prevalent bend direction or plane.According to one embodiment, such light guide 800 may include lenses 208that extend parallel to a prevalent bend direction or plane.

According to one embodiment, lenses 208 may be replaced by point-focuslenses, for example, such as those described in the '007 patent. Thepoint-focus lenses may be dome-shaped (e.g., spherical or elliptical) ormay be formed by Fresnel microstructures and configured as small-sizeFresnel lenses. According to one embodiment, point-focus lenses 208 maybe disposed in registration or alignment with at least some of lightextraction features 8 and may further be configured to have a focallength approximating thickness T₁ of sheet 10, thickness T₂ of sheet 20or a combined thickness T₁+T₂ such that two-dimensionally collimatedbeams of light may be formed when light guide 800 is illuminated by therespective edge-coupled light sources.

Flexible stepped light guide 800 may include as many sheets 20 (and asmany edge-coupled LEDs) as needed to form a one-dimensional or atwo-dimensional array of sections 54 and 55. It may be appreciated thatstepped light guide illumination system 900 may be configured toaccommodate a large two-dimensional array of LED sources and may stillmaintain the low prescribed thickness regardless of the overall size ofthe. As discussed above, stepped light guide 800 may include a repeatingpattern of sections 54 and 55 and also include a repeating pattern oflight extraction features 8 and 9 corresponding to such sections.Accordingly, flexible stepped light guide 800 may be manufactured in theform of a large, continuous sheet of a variable thickness, including alarge number of sections 54 and 55 with the associated light extractionfeatures. According to one embodiment, such continuous large-area sheetmay then be cut into smaller sheets, depending on the application. Forexample, it may be cut along a midline of one of the sections 55, thusproducing two smaller-size stepped light guides 800 having the samebasic structure, thickness and operation.

According to one embodiment, flexible stepped light guide illuminationsystem 900 is provided with light extraction features 8 and 9 that areprinted on respective surfaces 11 and 21, where the ink or paint (e.g.,a UV-curable white ink) used to print light extraction features 8 and 9(using ink jet printing or screen printing, for example) may includelight scattering particles immersed into a polymeric resin which remainsflexible and stretchable without cracking even when fully cured. Forexample, according to different implementations, the fully cured ink orpaint may have elongation at break of at least 25%, at least 50%, atleast 100%, and at least 150%.

According to one embodiment, light extraction 8 and/or 9 may be formedin surface 11 of sheet 10 in the areas where sheets 20 are attached tosheet 10, e.g., in sections 54 of FIG. 1 or FIG. 23 or in sections 52 ofFIG. 13. FIG. 32 schematically illustrates an exemplary embodiment ofthis type in which a plurality of light extraction features 8′ is formedin section 52 of surface 11 of sheet 10. Light extraction features 8′may be of the same type as light extraction features 8 and/or 9.Alternatively, light extraction features 8′ may differ from lightextraction features 8 and/or 9, for example, by the shape, compositionor a method of making. According to one embodiment, substantially theentire area of surface 11 may be covered with light extraction features8, 8′ and/or 9. For example, the pattern of light extraction features 8and 9 shown in FIG. 15 may be formed in surface 11.

Further details of operation of illumination systems shown in thedrawing figures as well as their possible variations will be apparentfrom the foregoing description of preferred embodiments. Although thedescription above contains many details, these should not be construedas limiting the scope of the invention but as merely providingillustrations of some of the presently preferred embodiments of thisinvention. Therefore, it will be appreciated that the scope of thepresent invention fully encompasses other embodiments which may becomeobvious to those skilled in the art, and that reference to an element inthe singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

Any numerical range recited herein is intended to include all sub-rangessubsumed therein. For example, a range of 1 to 10 is intended to includeall sub-ranges between and including the recited minimum value of 1 andthe recited maximum value of 10, that is, having a minimum value equalto or greater than 1 and a maximum value of equal to or less than 10,such as, for example, 3 to 6 or 2.5 to 8.5. Any maximum numericallimitation recited herein is intended to include all lower numericallimitations subsumed therein and any minimum numerical limitationrecited in this specification is intended to include all highernumerical limitations subsumed therein. Accordingly, Applicant reservesthe right to amend this specification, including the claims, toexpressly recite any sub-range subsumed within the ranges expresslyrecited herein. All such ranges are intended to be inherently describedin this specification such that amending to expressly recite any suchsubranges would comply with the requirements of 35 U.S.C. § 112, firstparagraph, and 35 U.S.C. § 132(a). Also, unless otherwise indicated, allnumerical parameters are to be understood as being prefaced and modifiedin all instances by the term “about”, in which the numerical parameterspossess the inherent variability characteristic of the underlyingmeasurement techniques used to determine the numerical value of theparameter.

What is claimed is:
 1. An illumination system, comprising: a firstplanar sheet of an optically transmissive material defined by a firstbroad-area surface configured for light output and an opposing secondbroad-area surface, the first planar sheet is configured for guidinglight along a prevalent plane of the first planar sheet; a plurality ofsecond planar sheets of an optically transmissive material attached tothe second broad-area surface and spaced apart from each other bynon-zero spacing distances; a first plurality of light extractionfeatures formed in the second broad-area surface of the first planarsheet in spaces between the plurality of second planar sheets; a secondplurality of light extraction features formed in a surface of at leastone of the plurality of second planar sheets; and a plurality of LEDsoptically coupled to an edge of at least one of the plurality of secondplanar sheets, wherein a distribution density of the first plurality oflight extraction features is different from a distribution density ofthe second plurality of light extraction features.
 2. An illuminationsystem as recited in claim 1, wherein a relative surface area of thefirst plurality of light extraction features is different than arelative surface area of the second plurality of light extractionfeatures.
 3. An illumination system as recited in claim 1, wherein arelative surface area of the first plurality of light extractionfeatures is greater than a relative surface area of the second pluralityof light extraction features at least by a factor of 1.2.
 4. Anillumination system as recited in claim 1, wherein an opticallytransmissive structure formed by the first and second planar sheets isadapted for being retained in a bent or curved configuration.
 5. Anillumination system as recited in claim 1, wherein each of the pluralityof second planar sheets is optically coupled to the first planar sheetwith refractive index matching.
 6. An illumination system as recited inclaim 1, comprising a sheet of a highly reflective material attached ordisposed in an immediate proximity to a portion of the second broad-areasurface in a space between adjacent ones of the plurality of secondplanar sheets.
 7. An illumination system as recited in claim 1,comprising a sheet of a highly reflective material attached or disposedin an immediate proximity to a surface of at least one of the pluralityof second planar sheets.
 8. An illumination system as recited in claim1, comprising a printed circuit board at least partially located withina space between a pair of adjacent ones of the plurality of secondplanar sheets.
 9. An illumination system as recited in claim 1, whereineach of said plurality of LEDs is a side-emitting LED attached to acommon heat-spreading substrate located below the plurality of secondplanar sheets and extending parallel to the prevalent plane.
 10. Anillumination system as recited in claim 1, comprising an opticallytransmissive light diffusing sheet adjacent or in a close proximity tothe first broad-area surface.
 11. An illumination system as recited inclaim 1, comprising a brightness enhancement film positioned on a sideof the first broad-area surface.
 12. An illumination system as recitedin claim 1, wherein said plurality of second planar sheets is arrangedinto an ordered two-dimensional array having rows and columns.
 13. Anillumination system as recited in claim 1, comprising a plurality oflinear cylindrical lenses formed in the first broad-area surface andlongitudinally extending perpendicular to the edge.
 14. An illuminationsystem as recited in claim 1, comprising a plurality of parallel ridgesformed in the first broad-area surface and longitudinally extendingperpendicular to the edge.
 15. An illumination system as recited inclaim 1, wherein a combined thickness of the first and second planarsheets is equal to or below 3 mm.
 16. An illumination system,comprising: a first planar sheet of an optically transmissive materialdefined by a first broad-area surface configured for light output and anopposing second broad-area surface, the first planar sheet is configuredfor guiding light along a prevalent plane of the first planar sheet; aplurality of second planar sheets of an optically transmissive materialattached to the second broad-area surface and spaced apart from eachother by non-zero spacing distances; a first plurality of lightextraction features formed in the second broad-area surface of the firstplanar sheet in spaces between the plurality of second planar sheets; asecond plurality of light extraction features formed in a surface of atleast one of the plurality of second planar sheets; and a plurality ofLEDs optically coupled to an edge of at least one of the plurality ofsecond planar sheets, wherein an optically transmissive structure formedby the first and second planar sheets is adapted for being retained in abent or curved configuration.
 17. An illumination system as recited inclaim 16, comprising a plurality of parallel ridges formed in the firstbroad-area surface and longitudinally extending perpendicular to theedge.
 18. An illumination system as recited in claim 16, wherein each ofsaid plurality of LEDs is a side-emitting LED attached to a commonheat-spreading substrate located below the plurality of second planarsheets and extending parallel to the prevalent plane.
 19. Anillumination system as recited in claim 16, wherein said plurality ofsecond planar sheets is arranged into an ordered two-dimensional arrayhaving rows and columns.
 20. An illumination system, comprising: a firstplanar sheet of an optically transmissive material defined by a firstbroad-area surface configured for light output and an opposing secondbroad-area surface, the first planar sheet is configured for guidinglight along a prevalent plane of the first planar sheet; a plurality ofsecond planar sheets of an optically transmissive material distributedover an area of the second broad-area surface and spaced apart from eachother by non-zero spacing distances; a first plurality of lightextraction features formed in the second broad-area surface of the firstplanar sheet in spaces between the plurality of second planar sheets; asecond plurality of light extraction features formed in a surface of atleast one of the plurality of second planar sheets; a first plurality ofLEDs optically coupled to an edge of the first planar sheet; and asecond plurality of LEDs optically coupled to an edge of at least one ofthe plurality of second planar sheets, wherein a distribution density ofthe first plurality of light extraction features is different from adistribution density of the second plurality of light extractionfeatures.